Image encoding and decoding method using bidirectional prediction, and image encoding and decoding apparatus

ABSTRACT

Disclosed is an image decoding method according to an embodiment, the image decoding method including: obtaining a first reference block and a second reference block, for bi-directional prediction of a current block; obtaining, from a bitstream, weight information for combining the first reference block with the second reference block; performing entropy decoding on the weight information to obtain a weight index; combining the first reference block with the second reference block according to a candidate value indicated by the weight index among candidate values included in a weight candidate group; and reconstructing the current block based on a result of the combining, wherein a first binary value corresponding to the weight index is entropy-decoded based on a context model, and the remaining binary value corresponding to the weight index is entropy-decoded by a bypass method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation application of U.S. application Ser. No.17/294,164 filed May 14, 2021, which is a National Stage ofInternational Application No. PCT/KR2019/015654 filed Nov. 15, 2019,which claims benefit of U.S. Provisional No. 62/768,193 filed on Nov.16, 2018 in the United States Patent and Trademark Office. Thedisclosures of which are incorporated herein by reference in theirentireties.

TECHNICAL FIELD

The disclosure relates to the image encoding and decoding field. Moreparticularly, the disclosure relates to an image encoding method andapparatus and an image decoding method and apparatus, usingbi-directional prediction.

BACKGROUND ART

In image encoding and decoding, an image is split into blocks, and eachblock is prediction-encoded and prediction-decoded through interprediction or intra prediction.

Intra prediction is a method of removing spatial redundancy in images tocompress the images, and inter prediction is a method of removingtemporal redundancy between images to compress the images. Arepresentative example of inter prediction is motion estimation coding.Motion estimation coding predicts blocks of a current image by using areference image. A reference block that is most similar to a currentblock is searched within a preset search range by using a presetevaluation function. The current block is predicted based on thereference block, and a prediction block generated as the predictedresult is subtracted from the current block to generate a residualblock. The residual block is then encoded. To more accurately performthe prediction, interpolation is performed on the reference image togenerate pixels in a sub pel unit that is smaller than an integer pelunit, and inter prediction is performed based on the pixels in the subpel unit.

In a codec, such as H.264 Advanced Video Coding (AVC) and HighEfficiency Video Coding (HEVC), motion vectors of previously encodedblocks adjacent to a current block or blocks included in previouslyencoded images are used as prediction motion vectors of the currentblock in order to predict a motion vector of the current block.Differential motion vectors, which are differences between motionvectors of the current block and the prediction motion vectors, aresignaled to a decoder side through a preset method.

DESCRIPTION OF EMBODIMENTS Technical Problem

According to a technical object, an image decoding apparatus and methodand an image encoding apparatus and method, according to an embodiment,may enable image encoding and decoding with a low bit rate by reducing asize of residual data.

Also, according to another technical object, an image decoding apparatusand method and an image encoding apparatus and method, according to anembodiment, may achieve simplification of entropy encoding and entropydecoding of data included in a bitstream.

Solution to Problem

According to an embodiment, an image decoding method usingbi-directional prediction includes: obtaining a first reference block ina first reference image and a second reference block in a secondreference image for bi-directional prediction of a current block;obtaining weight information for combing the first reference block withthe second reference block from a bitstream; performing entropy decodingon the weight information to obtain a weight index; combining the firstreference block with the second reference block according to a candidatevalue indicated by the weight index among candidate values included in aweight candidate group; and reconstructing the current block based on aresult of the combining, wherein a first binary value corresponding tothe weight index is entropy-decoded based on a context model, and theremaining binary value corresponding to the weight index isentropy-decoded by a bypass method.

Advantageous Effects of Disclosure

An image decoding apparatus and method and an image encoding apparatusand method, according to an embodiment, may encode and decode imageswith a low bit rate by reducing a size of residual data.

Also, an image decoding apparatus and method and an image encodingapparatus and method, according to an embodiment, may simplify entropyencode and entropy decode data included in a bitstream.

It should be noted that effects that can be achieved by the imagedecoding apparatus and method and the image encoding apparatus andmethod according to the embodiment are not limited to those describedabove, and other effects not mentioned will be apparent to one ofordinary skill in the art from the following descriptions.

BRIEF DESCRIPTION OF DRAWINGS

A brief description of each drawing is provided for better understandingof the drawings cited herein.

FIG. 1 is a block diagram of an image decoding apparatus according to anembodiment.

FIG. 2 is a block diagram of an image encoding apparatus according to anembodiment.

FIG. 3 illustrates a process, performed by an image decoding apparatus,of determining at least one coding unit by splitting a current codingunit, according to an embodiment.

FIG. 4 illustrates a process, performed by an image decoding apparatus,of determining at least one coding unit by splitting a non-square codingunit, according to an embodiment.

FIG. 5 illustrates a process, performed by an image decoding apparatus,of splitting a coding unit based on at least one of block shapeinformation and split shape mode information, according to anembodiment.

FIG. 6 illustrates a method, performed by an image decoding apparatus,of determining a preset coding unit from among an odd number of codingunits, according to an embodiment.

FIG. 7 illustrates an order of processing a plurality of coding unitswhen an image decoding apparatus determines the plurality of codingunits by splitting a current coding unit, according to an embodiment.

FIG. 8 illustrates a process, performed by an image decoding apparatus,of determining that a current coding unit is to be split into an oddnumber of coding units, when the coding units are not processable in apreset order, according to an embodiment.

FIG. 9 illustrates a process, performed by an image decoding apparatus,of determining at least one coding unit by splitting a first codingunit, according to an embodiment.

FIG. 10 illustrates that a shape into which a second coding unit issplittable is restricted when the second coding unit having a non-squareshape, which is determined when an image decoding apparatus splits afirst coding unit, satisfies a preset condition, according to anembodiment.

FIG. 11 illustrates a process, performed by an image decoding apparatus,of splitting a square coding unit when split shape mode informationindicates that the square coding unit is to not be split into foursquare coding units, according to an embodiment.

FIG. 12 illustrates that a processing order between a plurality ofcoding units may be changed depending on a process of splitting a codingunit, according to an embodiment.

FIG. 13 illustrates a process of determining a depth of a coding unit asa shape and size of the coding unit change, when the coding unit isrecursively split such that a plurality of coding units are determined,according to an embodiment.

FIG. 14 illustrates depths that are determinable based on shapes andsizes of coding units, and part indexes (PIDs) that are fordistinguishing the coding units, according to an embodiment.

FIG. 15 illustrates that a plurality of coding units are determinedbased on a plurality of preset data units included in a picture,according to an embodiment.

FIG. 16 illustrates a processing block used as a criterion fordetermining an order of determining reference coding units included in apicture, according to an embodiment.

FIG. 17 illustrates coding units of individual pictures, when theindividual pictures have different split shape combinations of codingunits, according to an embodiment.

FIG. 18 illustrates various shapes of coding units that can bedetermined based on split shape mode information that may be expressedwith a binary code, according to an embodiment.

FIG. 19 illustrates other shapes of coding units that may be determinedbased on split shape mode information that may be expressed with abinary code, according to an embodiment.

FIG. 20 is a block diagram of an image encoding and decoding system thatperforms loop filtering.

FIG. 21 is a block diagram illustrating a configuration of an imagedecoding apparatus according to an embodiment.

FIG. 22 illustrates a configuration of an entropy decoder shown in FIG.21 .

FIG. 23 is a reference table for determining context information of abinary string corresponding to a weight index.

FIG. 24 is a table showing candidate values included in a weightcandidate group, and weight indexes and binary strings corresponding tothe candidate values.

FIG. 25 is a view for describing bi-directional prediction of a currentblock.

FIG. 26 illustrates blocks temporally and spatially related to a currentblock.

FIG. 27 is a view for describing that indexes assigned to candidatevalues included in a weight candidate group may change according toaccumulative numbers of times the candidate values have been selected.

FIG. 28 is a table showing candidate values included in a weightcandidate group, and weight indexes and binary strings corresponding tothe candidate values.

FIG. 29 is a flowchart illustrating an image decoding method accordingto an embodiment.

FIG. 30 is a block diagram illustrating a configuration of an imageencoding apparatus according to an embodiment.

FIG. 31 illustrates a configuration of an entropy encoder shown in FIG.30 .

FIG. 32 is a flowchart illustrating an image encoding method accordingto an embodiment.

BEST MODE

According to an embodiment, an image decoding method usingbi-directional prediction includes: obtaining a first reference block ina first reference image and a second reference block in a secondreference image for bi-directional prediction of a current block;obtaining weight information for combing the first reference block withthe second reference block from a bitstream; obtaining a weight index byentropy decoding the weight information; combining the first referenceblock with the second reference block according to a candidate valueindicated by the weight index among candidate values included in aweight candidate group; and reconstructing the current block based on aresult of the combining, wherein a first binary value corresponding tothe weight index is entropy-decoded based on a context model, and theremaining binary value corresponding to the weight index isentropy-decoded by a bypass method.

The image decoding method may further include adaptively determining anumber of the candidate values included in the weight candidate group,based on at least one of a Picture Order Count (POC) of the firstreference image and a POC of the second reference image.

The adaptively determining of the number of the candidate values mayinclude determining the number of the candidate values included in theweight candidate group, based on at least one of a result of comparisonbetween the POC of the first reference image and a POC of a currentimage including the current block and a result of comparison between thePOC of the second reference image and the POC of the current image.

The number of the candidate values included in the weight candidategroup when the POC of the first reference image and the POC of thesecond reference image are smaller than or equal to the POC of thecurrent image may be different from the number of the candidate valuesincluded in the weight candidate group when the POC of the firstreference image and the POC of the second reference image are greaterthan the POC of the current image.

The number of binary values corresponding to the weight index may varyaccording to a value of the weight index.

The image decoding method may further include assigning indexes tocandidate values to be used for bi-directional prediction of the currentblock, wherein the combining of the first reference block with thesecond reference block includes combining the first reference block withthe second reference block, according to a candidate value to which anindex corresponding to the weight index is assigned.

The assigning of the indexes may include assigning indexes to thecandidate values to be used for bi-directional prediction of the currentblock, according to accumulative numbers of times which the candidatevalues have been respectively selected for bi-directional prediction ofprevious blocks.

The indexes may be assigned to the candidate values for each image, eachframe, or each tile.

The assigning of the indexes may include: selecting a previous imagehaving a same temporal layer as a temporal layer of the current image ina current Group of Picture (GOP); and assigning the indexes to thecandidate values to be used for bi-directional prediction of the currentblock, according to the accumulative numbers of times which thecandidate values have been respectively selected for bi-directionalprediction of previous blocks included in the selected previous image.

The assigning of the indexes may include assigning the indexes to thecandidate values to be used for bi-directional prediction of the currentblock, according to the accumulative numbers of times which thecandidate values have been respectively selected for previous blocksbi-directionally predicted by using the first reference image and thesecond reference image.

The combining of the first reference block with the second referenceblock may include: determining a pair value of the candidate valueindicated by the weight index; and applying one of the candidate valueand the pair value to the first reference block and applying theremaining one to the second reference block to thereby combine the firstreference block with the second reference block.

The pair value may be not included in the weight candidate group, andthe combining of the first reference block with the second referenceblock may include selecting a value that is to be applied to the firstreference block and the second reference block from among the candidatevalue and the pair value, based on a POC of the first reference imageand a POC of the second reference image.

According to an embodiment, an image decoding apparatus usingbi-directional prediction includes: an obtainer configured to obtain abitstream including weight information for bi-directional prediction ofa current block; an entropy decoder configured to obtain a weight indexby entropy decoding the weight information; and a prediction decoderconfigured to obtain a first reference block in a first reference imageand a second reference block in a second reference image forbi-directional prediction of the current block, combine the firstreference block with the second reference block according to a candidatevalue indicated by the weight index among candidate values included in aweight candidate group, and reconstruct the current block based on aresult of the combining, wherein a first binary value corresponding tothe weight index is entropy-decoded based on a context model, and theremaining binary value corresponding to the weight index isentropy-decoded by a bypass method.

According to an embodiment, an image encoding method usingbi-directional prediction includes: obtaining a first reference block ina first reference image and a second reference block in a secondreference image for bi-directional prediction of a current block;selecting a candidate value for combining the first reference block withthe second reference block from among candidate values included in aweight candidate group; performing entropy encoding on a weight indexindicating the selected candidate value; and generating a bitstreamincluding weight information obtained as a result of the entropyencoding, and residual data, wherein a first binary value correspondingto the weight index is entropy-encoded based on a context model, and theremaining binary value corresponding to the weight index isentropy-encoded by a bypass method.

Mode of Disclosure

As the disclosure allows for various changes and numerous embodiments,specific embodiments will be illustrated in the drawings and describedin detail in the written description. However, this is not intended tolimit embodiments to particular modes of practice, and it is to beappreciated that the disclosure includes all changes, equivalents, andsubstitutes that do not depart from the spirit and technical scope ofembodiments.

In the description of embodiments, certain detailed explanations ofrelated art are omitted when it is deemed that they may unnecessarilyobscure the essence of the disclosure. Also, numbers (for example, afirst, a second, and the like) used in the description of thespecification are merely identifier codes for distinguishing onecomponent from another.

Also, in the present specification, it will be understood that whencomponents are “connected” or “coupled” to each other, the componentsmay be directly connected or coupled to each other, but mayalternatively be connected or coupled to each other with an interveningcomponent therebetween, unless specified otherwise.

In the present specification, regarding a component represented as a“portion (unit)” or a “module”, two or more components may be combinedinto one component or one component may be divided into two or morecomponents according to subdivided functions. In addition, eachcomponent described hereinafter may additionally perform some or all offunctions performed by another component, in addition to main functionsof itself, and some of the main functions of each component may beperformed entirely by another component.

Also, in the present specification, an ‘image’ or a ‘picture’ may denotea still image. Also, the ‘image’ or ‘picture’ may denote a frameconstituting a video, or a video itself.

Also, in the present specification, a ‘sample’ or ‘signal’ means, asdata assigned to a sampling location of an image, data to be processed.For example, pixel values on a spatial-domain image and transformcoefficients on a transform domain may be samples. A unit including suchat least one sample may be defined as a block.

Hereinafter, an image encoding method and apparatus and an imagedecoding method and apparatus, based on a coding unit and a transformunit of a tree structure according to an embodiment will be disclosedwith reference to FIGS. 1 to 20 .

FIG. 1 is a block diagram of an image decoding apparatus 100 accordingto an embodiment.

The image decoding apparatus 100 may include a bitstream obtainer 110and a decoder 120. The bitstream obtainer 110 and the decoder 120 mayinclude at least one processor. Also, the bitstream obtainer 110 and thedecoder 120 may include a memory storing instructions that are executedby the at least one processor.

The bitstream obtainer 110 may receive a bitstream. The bitstream mayinclude information resulting from image encoding by an image encodingapparatus 200 which will be described later. Also, the bitstream may betransmitted from the image encoding apparatus 200. The image decodingapparatus 100 may be connected to the image encoding apparatus 200 in awired or wireless manner, and the bitstream obtainer 110 may receive abitstream in a wired or wireless manner. The bitstream obtainer 110 mayreceive a bitstream from a storage medium, such as optical media, a harddisk, etc. The decoder 120 may reconstruct an image based on informationobtained from the received bitstream. The decoder 120 may obtain asyntax element for reconstructing an image from the bitstream. Thedecoder 120 may reconstruct the image based on the syntax element.

The operation of the image decoding apparatus 100 will be described indetail below. The bitstream obtainer 110 may receive a bitstream.

The image decoding apparatus 100 may perform an operation of obtaining abin string corresponding to a split shape mode of a coding unit from thebitstream. Then, the image decoding apparatus 100 may perform anoperation of determining a split rule of a coding unit. Also, the imagedecoding apparatus 100 may perform an operation of splitting a codingunit into a plurality of coding units, based on at least one of the binstring corresponding to the split shape mode and the split rule. Theimage decoding apparatus 100 may determine a first range which is anallowable size range of a coding unit, according to a ratio of a heightto a width of the coding unit, in order to determine the split rule. Theimage decoding apparatus 100 may determine a second range which is anallowable size range of a coding unit, according to a split shape modeof the coding unit, in order to determine the split rule.

Hereinafter, splitting of a coding unit will be described in detailaccording to an embodiment of the disclosure.

First, one picture may be split into one or more slices or one or moretiles. One slice or one tile may be a sequence of one or more largestcoding units (coding tree units (CTUs)). There is a largest coding block(coding tree block (CTB)) conceptually compared to a largest coding unit(CTU).

The largest coding block (CTB) denotes an N×N block including N×Nsamples (where N is an integer). Each color component may be split intoone or more largest coding blocks.

When a picture has three sample arrays (sample arrays for Y, Cr, and Cbcomponents), a largest coding unit (CTU) includes a largest coding blockof a luma sample, two corresponding largest coding blocks of chromasamples, and syntax structures used to encode the luma sample and thechroma samples. When a picture is a monochrome picture, a largest codingunit includes a largest coding block of a monochrome sample and syntaxstructures used to encode the monochrome samples. When a picture is apicture encoded in color planes separated according to color components,a largest coding unit includes syntax structures used to encode thepicture and samples of the picture.

One largest coding block (CTB) may be split into M×N coding blocksincluding M×N samples (M and N are integers).

When a picture has sample arrays for Y, Cr, and Cb components, a codingunit (CU) includes a coding block of a luma sample, two correspondingcoding blocks of chroma samples, and syntax structures used to encodethe luma sample and the chroma samples. When a picture is a monochromepicture, a coding unit includes a coding block of a monochrome sampleand syntax structures used to encode the monochrome samples. When apicture is a picture encoded in color planes separated according tocolor components, a coding unit includes syntax structures used toencode the picture and samples of the picture.

As described above, a largest coding block and a largest coding unit areconceptually distinguished from each other, and a coding block and acoding unit are conceptually distinguished from each other. That is, a(largest) coding unit refers to a data structure including a (largest)coding block including a corresponding sample and a syntax structurecorresponding to the (largest) coding block. However, because it isunderstood by one of ordinary skill in the art that a (largest) codingunit or a (largest) coding block refers to a block of a preset sizeincluding a preset number of samples, a largest coding block and alargest coding unit, or a coding block and a coding unit are mentionedin the following specification without being distinguished unlessotherwise described.

An image may be split into largest coding units (CTUs). A size of eachlargest coding unit may be determined based on information obtained froma bitstream. A shape of each largest coding unit may be a square shapeof the same size. However, the embodiment is not limited thereto.

For example, information about a maximum size of a luma coding block maybe obtained from a bitstream. For example, the maximum size of the lumacoding block indicated by the information about the maximum size of theluma coding block may be one of 4×4, 8×8, 16×16, 32×32, 64×64, 128×128,and 256×256.

For example, information about a luma block size difference and amaximum size of a luma coding block that may be split into two may beobtained from a bitstream. The information about the luma block sizedifference may refer to a size difference between a luma largest codingunit and a largest luma coding block that may be split into two.Accordingly, when the information about the maximum size of the lumacoding block that may be split into two and the information about theluma block size difference obtained from the bitstream are combined witheach other, a size of the luma largest coding unit may be determined. Asize of a chroma largest coding unit may be determined by using the sizeof the luma largest coding unit. For example, when a Y:Cb:Cr ratio is4:2:0 according to a color format, a size of a chroma block may be halfa size of a luma block, and a size of a chroma largest coding unit maybe half a size of a luma largest coding unit.

According to an embodiment, because information about a maximum size ofa luma coding block that is binary splittable is obtained from abitstream, the maximum size of the luma coding block that is binarysplittable may be variably determined. In contrast, a maximum size of aluma coding block that is ternary splittable may be fixed. For example,the maximum size of the luma coding block that is ternary splittable inan I-slice may be 32×32, and the maximum size of the luma coding blockthat is ternary splittable in a P-slice or a B-slice may be 64×64.

Also, a largest coding unit may be hierarchically split into codingunits based on split shape mode information obtained from a bitstream.At least one of information indicating whether quad splitting isperformed, information indicating whether multi-splitting is performed,split direction information, and split type information may be obtainedas the split shape mode information from the bitstream.

For example, the information indicating whether quad splitting isperformed may indicate whether a current coding unit is quad split(QUAD_SPLIT) or not.

When the current coding unit is not quad split, the informationindicating whether multi-splitting is performed may indicate whether thecurrent coding unit is no longer split (NO_SPLIT) or binary/ternarysplit.

When the current coding unit is binary split or ternary split, the splitdirection information indicates that the current coding unit is split inone of a horizontal direction and a vertical direction.

When the current coding unit is split in the horizontal direction or thevertical direction, the split type information indicates that thecurrent coding unit is binary split or ternary split.

A split mode of the current coding unit may be determined according tothe split direction information and the split type information. A splitmode when the current coding unit is binary split in the horizontaldirection may be determined to be a binary horizontal split mode(SPLIT_BT_HOR), a split mode when the current coding unit is ternarysplit in the horizontal direction may be determined to be a ternaryhorizontal split mode (SPLIT_TT_HOR), a split mode when the currentcoding unit is binary split in the vertical direction may be determinedto be a binary vertical split mode (SPLIT_BT_VER), and a split mode whenthe current coding unit is ternary split in the vertical direction maybe determined to be a ternary vertical split mode (SPLIT_TT_VER).

The image decoding apparatus 100 may obtain, from the bitstream, thesplit shape mode information from one bin string. A form of thebitstream received by the image decoding apparatus 100 may include fixedlength binary code, unary code, truncated unary code, predeterminedbinary code, or the like. The bin string is information in a binarynumber. The bin string may include at least one bit. The image decodingapparatus 100 may obtain the split shape mode information correspondingto the bin string, based on the split rule. The image decoding apparatus100 may determine whether to quad split a coding unit, whether not tosplit a coding unit, a split direction, and a split type, based on onebin string.

The coding unit may be smaller than or the same as the largest codingunit. For example, because a largest coding unit is a coding unit havinga maximum size, the largest coding unit is one of coding units. Whensplit shape mode information about a largest coding unit indicates thatsplitting is not performed, a coding unit determined in the largestcoding unit has the same size as that of the largest coding unit. Whensplit shape mode information about a largest coding unit indicates thatsplitting is performed, the largest coding unit may be split into codingunits. Also, when split shape mode information about a coding unitindicates that splitting is performed, the coding unit may be split intosmaller coding units. However, the splitting of the image is not limitedthereto, and the largest coding unit and the coding unit may not bedistinguished. The splitting of the coding unit will be described indetail with reference to FIGS. 3 to 16 .

Also, one or more prediction blocks for prediction may be determinedfrom a coding unit. The prediction block may be the same as or smallerthan the coding unit. Also, one or more transform blocks fortransformation may be determined from a coding unit. The transform blockmay be equal to or smaller than the coding unit.

The shapes and sizes of the transform block and prediction block may notbe related to each other.

In another embodiment, prediction may be performed by using a codingunit as a prediction unit. Also, transformation may be performed byusing a coding unit as a transform block.

The splitting of the coding unit will be described in detail withreference to FIGS. 3 to 16 . A current block and an adjacent block ofthe disclosure may indicate one of the largest coding unit, the codingunit, the prediction block, and the transform block. Also, the currentblock of the current coding unit is a block that is currently beingdecoded or encoded or a block that is currently being split. Theadjacent block may be a block reconstructed before the current block.The adjacent block may be adjacent to the current block spatially ortemporally. The adjacent block may be located at one of the lower left,left, upper left, top, upper right, right, lower right of the currentblock.

FIG. 3 illustrates a process, performed by the image decoding apparatus100, of determining at least one coding unit by splitting a currentcoding unit, according to an embodiment.

A block shape may include 4N×4N, 4N×2N, 2N×4N, 4N×N, N×4N, 32N×N, N×32N,16N×N, N×16N, 8N×N, or N×8N. Here, N may be a positive integer. Blockshape information is information indicating at least one of a shape, adirection, a ratio of width and height, or size of a coding unit.

The shape of the coding unit may include a square and a non-square. Whenthe lengths of the width and height of the coding unit are the same(i.e., when the block shape of the coding unit is 4N×4N), the imagedecoding apparatus 100 may determine the block shape information of thecoding unit as a square. The image decoding apparatus 100 may determinethe shape of the coding unit to be a non-square.

When the width and the height of the coding unit are different from eachother (i.e., when the block shape of the coding unit is 4N×2N, 2N×4N,4N×N, N×4N, 32N×N, N×32N, 16N×N, N×16N, 8N×N, or N×8N), the imagedecoding apparatus 100 may determine the block shape information of thecoding unit as a non-square shape. When the shape of the coding unit isnon-square, the image decoding apparatus 100 may determine the ratio ofthe width and height among the block shape information of the codingunit to be at least one of 1:2, 2:1, 1:4, 4:1, 1:8, 8:1, 1:16, 16:1,1:32, and 32:1. Also, the image decoding apparatus 100 may determinewhether the coding unit is in a horizontal direction or a verticaldirection, based on the length of the width and the length of the heightof the coding unit. Also, the image decoding apparatus 100 may determinethe size of the coding unit, based on at least one of the length of thewidth, the length of the height, or the area of the coding unit.

According to an embodiment, the image decoding apparatus 100 maydetermine the shape of the coding unit by using the block shapeinformation, and may determine a splitting method of the coding unit byusing the split shape mode information. That is, a coding unit splittingmethod indicated by the split shape mode information may be determinedbased on a block shape indicated by the block shape information used bythe image decoding apparatus 100.

The image decoding apparatus 100 may obtain the split shape modeinformation from a bitstream. However, an embodiment is not limitedthereto, and the image decoding apparatus 100 and the image encodingapparatus 200 may determine pre-agreed split shape mode information,based on the block shape information. The image decoding apparatus 100may determine the pre-agreed split shape mode information with respectto a largest coding unit or a minimum coding unit. For example, theimage decoding apparatus 100 may determine split shape mode informationwith respect to the largest coding unit to be a quad split. Also, theimage decoding apparatus 100 may determine split shape mode informationregarding the smallest coding unit to be “not to perform splitting”. Inparticular, the image decoding apparatus 100 may determine the size ofthe largest coding unit to be 256×256. The image decoding apparatus 100may determine the pre-agreed split shape mode information to be a quadsplit. The quad split is a split shape mode in which the width and theheight of the coding unit are both bisected. The image decodingapparatus 100 may obtain a coding unit of a 128×128 size from thelargest coding unit of a 256×256 size, based on the split shape modeinformation. Also, the image decoding apparatus 100 may determine thesize of the smallest coding unit to be 4×4. The image decoding apparatus100 may obtain split shape mode information indicating “not to performsplitting” with respect to the smallest coding unit.

According to an embodiment, the image decoding apparatus 100 may use theblock shape information indicating that the current coding unit has asquare shape. For example, the image decoding apparatus 100 maydetermine whether not to split a square coding unit, whether tovertically split the square coding unit, whether to horizontally splitthe square coding unit, or whether to split the square coding unit intofour coding units, based on the split shape mode information. Referringto FIG. 3 , when the block shape information of a current coding unit300 indicates a square shape, the decoder 120 may not split a codingunit 310 a having the same size as the current coding unit 300, based onthe split shape mode information indicating not to perform splitting, ormay determine coding units 310 b, 310 c, 310 d, 310 e, or 310 f splitbased on the split shape mode information indicating a preset splittingmethod.

Referring to FIG. 3 , according to an embodiment, the image decodingapparatus 100 may determine two coding units 310 b obtained by splittingthe current coding unit 300 in a vertical direction, based on the splitshape mode information indicating to perform splitting in a verticaldirection. The image decoding apparatus 100 may determine two codingunits 310 c obtained by splitting the current coding unit 300 in ahorizontal direction, based on the split shape mode informationindicating to perform splitting in a horizontal direction. The imagedecoding apparatus 100 may determine four coding units 310 d obtained bysplitting the current coding unit 300 in vertical and horizontaldirections, based on the split shape mode information indicating toperform splitting in vertical and horizontal directions. According to anembodiment, the image decoding apparatus 100 may determine three codingunits 310 e obtained by splitting the current coding unit 300 in avertical direction, based on the split shape mode information indicatingto perform ternary splitting in a vertical direction. The image decodingapparatus 100 may determine three coding units 310 f obtained bysplitting the current coding unit 300 in a horizontal direction, basedon the split shape mode information indicating to perform ternarysplitting in a horizontal direction. However, splitting methods of thesquare coding unit are not limited to the above-described methods, andthe split shape mode information may indicate various methods. Presetsplitting methods of splitting the square coding unit will be describedin detail below in relation to various embodiments.

FIG. 4 illustrates a process, performed by the image decoding apparatus100, of determining at least one coding unit by splitting a non-squarecoding unit, according to an embodiment.

According to an embodiment, the image decoding apparatus 100 may useblock shape information indicating that a current coding unit has anon-square shape. The image decoding apparatus 100 may determine whethernot to split the non-square current coding unit or whether to split thenon-square current coding unit by using a preset splitting method, basedon split shape mode information. Referring to FIG. 4 , when the blockshape information of a current coding unit 400 or 450 indicates anon-square shape, the image decoding apparatus 100 may determine acoding unit 410 or 460 having the same size as the current coding unit400 or 450, based on the split shape mode information indicating not toperform splitting, or may determine coding units 420 a and 420 b, 430 ato 430 c, 470 a and 470 b, or 480 a to 480 c split based on the splitshape mode information indicating a preset splitting method. Presetsplitting methods of splitting a non-square coding unit will bedescribed in detail below in relation to various embodiments.

According to an embodiment, the image decoding apparatus 100 maydetermine a splitting method of a coding unit by using the split shapemode information and, in this case, the split shape mode information mayindicate the number of one or more coding units generated by splitting acoding unit. Referring to FIG. 4 , when the split shape mode informationindicates to split the current coding unit 400 or 450 into two codingunits, the image decoding apparatus 100 may determine two coding units420 a and 420 b, or 470 a and 470 b included in the current coding unit400 or 450, by splitting the current coding unit 400 or 450 based on thesplit shape mode information.

According to an embodiment, when the image decoding apparatus 100 splitsthe non-square current coding unit 400 or 450 based on the split shapemode information, the image decoding apparatus 100 may consider thelocation of a long side of the non-square current coding unit 400 or 450to split a current coding unit. For example, the image decodingapparatus 100 may determine a plurality of coding units by splitting thecurrent coding unit 400 or 450 in a direction of splitting a long sideof the current coding unit 400 or 450, in consideration of the shape ofthe current coding unit 400 or 450.

According to an embodiment, when the split shape mode informationindicates to split (ternary split) a coding unit into an odd number ofblocks, the image decoding apparatus 100 may determine an odd number ofcoding units included in the current coding unit 400 or 450. Forexample, when the split shape mode information indicates to split thecurrent coding unit 400 or 450 into three coding units, the imagedecoding apparatus 100 may split the current coding unit 400 or 450 intothree coding units 430 a, 430 b, and 430 c, or 480 a, 480 b, and 480 c.

According to an embodiment, a ratio of the width and height of thecurrent coding unit 400 or 450 may be 4:1 or 1:4. When the ratio of thewidth and height is 4:1, the block shape information may indicate ahorizontal direction because the length of the width is longer than thelength of the height. When the ratio of the width and height is 1:4, theblock shape information may indicate a vertical direction because thelength of the width is shorter than the length of the height. The imagedecoding apparatus 100 may determine to split a current coding unit intoan odd number of blocks, based on the split shape mode information.Also, the image decoding apparatus 100 may determine a split directionof the current coding unit 400 or 450, based on the block shapeinformation of the current coding unit 400 or 450. For example, when thecurrent coding unit 400 is in the vertical direction, the image decodingapparatus 100 may determine the coding units 430 a, 430 b, and 430 c bysplitting the current coding unit 400 in the horizontal direction. Also,when the current coding unit 450 is in the horizontal direction, theimage decoding apparatus 100 may determine the coding units 480 a, 480b, and 480 c by splitting the current coding unit 450 in the verticaldirection.

According to an embodiment, the image decoding apparatus 100 maydetermine an odd number of coding units included in the current codingunit 400 or 450, and not all the determined coding units may have thesame size. For example, a preset coding unit 430 b or 480 b from amongthe determined odd number of coding units 430 a, 430 b, and 430 c, or480 a, 480 b, and 480 c may have a size different from the size of theother coding units 430 a and 430 c, or 480 a and 480 c. That is, codingunits which may be determined by splitting the current coding unit 400or 450 may have multiple sizes and, in some cases, all of the odd numberof coding units 430 a, 430 b, and 430 c, or 480 a, 480 b, and 480 c mayhave different sizes.

According to an embodiment, when the split shape mode informationindicates to split a coding unit into the odd number of blocks, theimage decoding apparatus 100 may determine the odd number of codingunits included in the current coding unit 400 or 450, and moreover, mayput a preset restriction on at least one coding unit from among the oddnumber of coding units generated by splitting the current coding unit400 or 450. Referring to FIG. 4 , the image decoding apparatus 100 mayset a decoding process regarding the coding unit 430 b or 480 b locatedat the center among the three coding units 430 a, 430 b, and 430 c, or480 a, 480 b, and 480 c generated as the current coding unit 400 or 450is split to be different from that of the other coding units 430 a and430 c, or 480 a and 480 c. For example, the image decoding apparatus 100may restrict the coding unit 430 b or 480 b at the center location to beno longer split or to be split only a preset number of times, unlike theother coding units 430 a and 430 c, or 480 a and 480 c.

FIG. 5 illustrates a process, performed by the image decoding apparatus100, of splitting a coding unit based on at least one of block shapeinformation and split shape mode information, according to anembodiment.

According to an embodiment, the image decoding apparatus 100 maydetermine to split or to not split a square first coding unit 500 intocoding units, based on at least one of the block shape information andthe split shape mode information. According to an embodiment, when thesplit shape mode information indicates to split the first coding unit500 in a horizontal direction, the image decoding apparatus 100 maydetermine a second coding unit 510 by splitting the first coding unit500 in a horizontal direction. A first coding unit, a second codingunit, and a third coding unit used according to an embodiment are termsused to understand a relation before and after splitting a coding unit.For example, a second coding unit may be determined by splitting a firstcoding unit, and a third coding unit may be determined by splitting thesecond coding unit. It will be understood that the relation of the firstcoding unit, the second coding unit, and the third coding unit followsthe above descriptions.

According to an embodiment, the image decoding apparatus 100 maydetermine to split or to not split the determined second coding unit 510into coding units, based on the split shape mode information. Referringto FIG. 5 , the image decoding apparatus 100 may split the non-squaresecond coding unit 510, which is determined by splitting the firstcoding unit 500, into one or more third coding units 520 a, 520 b, 520c, and 520 d based on at least one of the split shape mode informationand the split shape mode information, or may not split the non-squaresecond coding unit 510. The image decoding apparatus 100 may obtain thesplit shape mode information, and may obtain a plurality ofvarious-shaped second coding units (e.g., 510) by splitting the firstcoding unit 500, based on the obtained split shape mode information, andthe second coding unit 510 may be split by using a splitting method ofthe first coding unit 500 based on the split shape mode information.According to an embodiment, when the first coding unit 500 is split intothe second coding units 510 based on the split shape mode information ofthe first coding unit 500, the second coding unit 510 may also be splitinto the third coding units (e.g., 520 a, or 520 b, 520 c, and 520 d)based on the split shape mode information of the second coding unit 510.That is, a coding unit may be recursively split based on the split shapemode information of each coding unit. Therefore, a square coding unitmay be determined by splitting a non-square coding unit, and anon-square coding unit may be determined by recursively splitting thesquare coding unit.

Referring to FIG. 5 , a preset coding unit (e.g., a coding unit locatedat a center location, or a square coding unit) from among an odd numberof third coding units 520 b, 520 c, and 520 d determined by splittingthe non-square second coding unit 510 may be recursively split.According to an embodiment, the square third coding unit 520 c fromamong the odd number of third coding units 520 b, 520 c, and 520 d maybe split in a horizontal direction into a plurality of fourth codingunits. A non-square fourth coding unit 530 b or 530 d from among theplurality of fourth coding units 530 a, 530 b, 530 c, and 530 d may bere-split into a plurality of coding units. For example, the non-squarefourth coding unit 530 b or 530 d may be re-split into an odd number ofcoding units. A method that may be used to recursively split a codingunit will be described below in relation to various embodiments.

According to an embodiment, the image decoding apparatus 100 may spliteach of the third coding units 520 a, or 520 b, 520 c, and 520 d intocoding units, based on the split shape mode information. Also, the imagedecoding apparatus 100 may determine to not split the second coding unit510 based on the split shape mode information. According to anembodiment, the image decoding apparatus 100 may split the non-squaresecond coding unit 510 into the odd number of third coding units 520 b,520 c, and 520 d. The image decoding apparatus 100 may put a presetrestriction on a preset third coding unit from among the odd number ofthird coding units 520 b, 520 c, and 520 d. For example, the imagedecoding apparatus 100 may restrict the third coding unit 520 c at acenter location from among the odd number of third coding units 520 b,520 c, and 520 d to be no longer split or to be split a settable numberof times.

Referring to FIG. 5 , the image decoding apparatus 100 may restrict thethird coding unit 520 c, which is at the center location from among theodd number of third coding units 520 b, 520 c, and 520 d included in thenon-square second coding unit 510, to be no longer split, to be split byusing a preset splitting method (e.g., split into only four coding unitsor split by using a splitting method of the second coding unit 510), orto be split only a preset number of times (e.g., split only n times(where n>0)). However, the restrictions on the third coding unit 520 cat the center location are not limited to the above-described examples,and may include various restrictions for decoding the third coding unit520 c at the center location differently from the other third codingunits 520 b and 520 d.

According to an embodiment, the image decoding apparatus 100 may obtainthe split shape mode information, which is used to split a currentcoding unit, from a preset location in the current coding unit.

FIG. 6 illustrates a method, performed by the image decoding apparatus100, of determining a preset coding unit from among an odd number ofcoding units, according to an embodiment.

Referring to FIG. 6 , split shape mode information of a current codingunit 600 or 650 may be obtained from a sample of a preset location(e.g., a sample 640 or 690 of a center location) from among a pluralityof samples included in the current coding unit 600 or 650. However, thepreset location in the current coding unit 600, from which at least onepiece of the split shape mode information may be obtained, is notlimited to the center location in FIG. 6 , and may include variouslocations included in the current coding unit 600 (e.g., top, bottom,left, right, upper left, lower left, upper right, lower right locations,or the like). The image decoding apparatus 100 may obtain the splitshape mode information from the preset location and may determine tosplit or to not split the current coding unit into various-shaped andvarious-sized coding units.

According to an embodiment, when the current coding unit is split into apreset number of coding units, the image decoding apparatus 100 mayselect one of the coding units. Various methods may be used to selectone of a plurality of coding units, as will be described below inrelation to various embodiments.

According to an embodiment, the image decoding apparatus 100 may splitthe current coding unit into a plurality of coding units, and maydetermine a coding unit at a preset location.

According to an embodiment, image decoding apparatus 100 may useinformation indicating locations of the odd number of coding units, todetermine a coding unit at a center location from among the odd numberof coding units. Referring to FIG. 6 , the image decoding apparatus 100may determine the odd number of coding units 620 a, 620 b, and 620 c orthe odd number of coding units 660 a, 660 b, and 660 c by splitting thecurrent coding unit 600 or the current coding unit 650. The imagedecoding apparatus 100 may determine the middle coding unit 620 b or themiddle coding unit 660 b by using information about the locations of theodd number of coding units 620 a, 620 b, and 620 c or the odd number ofcoding units 660 a, 660 b, and 660 c. For example, the image decodingapparatus 100 may determine the coding unit 620 b of the center locationby determining the locations of the coding units 620 a, 620 b, and 620 cbased on information indicating locations of preset samples included inthe coding units 620 a, 620 b, and 620 c. In detail, the image decodingapparatus 100 may determine the coding unit 620 b at the center locationby determining the locations of the coding units 620 a, 620 b, and 620 cbased on information indicating locations of upper-left samples 630 a,630 b, and 630 c of the coding units 620 a, 620 b, and 620 c.

According to an embodiment, the information indicating the locations ofthe upper-left samples 630 a, 630 b, and 630 c, which are included inthe coding units 620 a, 620 b, and 620 c, respectively, may includeinformation about locations or coordinates of the coding units 620 a,620 b, and 620 c in a picture. According to an embodiment, theinformation indicating the locations of the upper-left samples 630 a,630 b, and 630 c, which are included in the coding units 620 a, 620 b,and 620 c, respectively, may include information indicating widths orheights of the coding units 620 a, 620 b, and 620 c included in thecurrent coding unit 600, and the widths or heights may correspond toinformation indicating differences between the coordinates of the codingunits 620 a, 620 b, and 620 c in the picture. That is, the imagedecoding apparatus 100 may determine the coding unit 620 b at the centerlocation by directly using the information about the locations orcoordinates of the coding units 620 a, 620 b, and 620 c in the picture,or by using the information about the widths or heights of the codingunits, which correspond to the difference values between thecoordinates.

According to an embodiment, information indicating the location of theupper-left sample 630 a of the upper coding unit 620 a may includecoordinates (xa, ya), information indicating the location of theupper-left sample 630 b of the center coding unit 620 b may includecoordinates (xb, yb), and information indicating the location of theupper-left sample 630 c of the lower coding unit 620 c may includecoordinates (xc, yc). The image decoding apparatus 100 may determine themiddle coding unit 620 b by using the coordinates of the upper-leftsamples 630 a, 630 b, and 630 c which are included in the coding units620 a, 620 b, and 620 c, respectively. For example, when the coordinatesof the upper-left samples 630 a, 630 b, and 630 c are sorted in anascending or descending order, the coding unit 620 b including thecoordinates (xb, yb) of the sample 630 b at a center location may bedetermined as a coding unit at a center location from among the codingunits 620 a, 620 b, and 620 c determined by splitting the current codingunit 600. However, the coordinates indicating the locations of theupper-left samples 630 a, 630 b, and 630 c may include coordinatesindicating absolute locations in the picture, or may use coordinates(dxb, dyb) indicating a relative location of the upper-left sample 630 bof the middle coding unit 620 b and coordinates (dxc, dyc) indicating arelative location of the upper-left sample 630 c of the lower codingunit 620 c with reference to the location of the upper-left sample 630 aof the upper coding unit 620 a. A method of determining a coding unit ata preset location by using coordinates of a sample included in thecoding unit, as information indicating a location of the sample, is notlimited to the above-described method, and may include variousarithmetic methods capable of using the coordinates of the sample.

According to an embodiment, the image decoding apparatus 100 may splitthe current coding unit 600 into a plurality of coding units 620 a, 620b, and 620 c, and may select one of the coding units 620 a, 620 b, and620 c based on a preset criterion. For example, the image decodingapparatus 100 may select the coding unit 620 b, which has a sizedifferent from that of the others, from among the coding units 620 a,620 b, and 620 c.

According to an embodiment, the image decoding apparatus 100 maydetermine the width or height of each of the coding units 620 a, 620 b,and 620 c by using the coordinates (xa, ya) that is the informationindicating the location of the upper-left sample 630 a of the uppercoding unit 620 a, the coordinates (xb, yb) that is the informationindicating the location of the upper-left sample 630 b of the middlecoding unit 620 b, and the coordinates (xc, yc) that are the informationindicating the location of the upper-left sample 630 c of the lowercoding unit 620 c. The image decoding apparatus 100 may determine therespective sizes of the coding units 620 a, 620 b, and 620 c by usingthe coordinates (xa, ya), (xb, yb), and (xc, yc) indicating thelocations of the coding units 620 a, 620 b, and 620 c. According to anembodiment, the image decoding apparatus 100 may determine the width ofthe upper coding unit 620 a to be the width of the current coding unit600. The image decoding apparatus 100 may determine the height of theupper coding unit 620 a to be yb-ya. According to an embodiment, theimage decoding apparatus 100 may determine the width of the middlecoding unit 620 b to be the width of the current coding unit 600. Theimage decoding apparatus 100 may determine the height of the middlecoding unit 620 b to be yc-yb. According to an embodiment, the imagedecoding apparatus 100 may determine the width or height of the lowercoding unit 620 c by using the width or height of the current codingunit 600 or the widths or heights of the upper and middle coding units620 a and 620 b. The image decoding apparatus 100 may determine a codingunit, which has a size different from that of the others, based on thedetermined widths and heights of the coding units 620 a, 620 b, and 620c. Referring to FIG. 6 , the image decoding apparatus 100 may determinethe middle coding unit 620 b, which has a size different from the sizeof the upper and lower coding units 620 a and 620 c, as the coding unitof the preset location. However, the above-described method, performedby the image decoding apparatus 100, of determining a coding unit havinga size different from the size of the other coding units merelycorresponds to an example of determining a coding unit at a presetlocation by using the sizes of coding units, which are determined basedon coordinates of samples, and thus various methods of determining acoding unit at a preset location by comparing the sizes of coding units,which are determined based on coordinates of preset samples, may beused.

The image decoding apparatus 100 may determine the width or height ofeach of the coding units 660 a, 660 b, and 660 c by using thecoordinates (xd, yd) that are information indicating the location of anupper-left sample 670 a of the left coding unit 660 a, the coordinates(xe, ye) that are information indicating the location of an upper-leftsample 670 b of the middle coding unit 660 b, and the coordinates (xf,yf) that are information indicating a location of the upper-left sample670 c of the right coding unit 660 c. The image decoding apparatus 100may determine the respective sizes of the coding units 660 a, 660 b, and660 c by using the coordinates (xd, yd), (xe, ye), and (xf, yf)indicating the locations of the coding units 660 a, 660 b, and 660 c.

According to an embodiment, the image decoding apparatus 100 maydetermine the width of the left coding unit 660 a to be xe−xd. The imagedecoding apparatus 100 may determine the height of the left coding unit660 a to be the height of the current coding unit 650. According to anembodiment, the image decoding apparatus 100 may determine the width ofthe middle coding unit 660 b to be xf−xe. The image decoding apparatus100 may determine the height of the middle coding unit 660 b to be theheight of the current coding unit 650. According to an embodiment, theimage decoding apparatus 100 may determine the width or height of theright coding unit 660 c by using the width or height of the currentcoding unit 650 or the widths or heights of the left and middle codingunits 660 a and 660 b. The image decoding apparatus 100 may determine acoding unit, which has a size different from that of the others, basedon the determined widths and heights of the coding units 660 a, 660 b,and 660 c. Referring to FIG. 6 , the image decoding apparatus 100 maydetermine the middle coding unit 660 b, which has a size different fromthe sizes of the left and right coding units 660 a and 660 c, as thecoding unit of the preset location. However, the above-described method,performed by the image decoding apparatus 100, of determining a codingunit having a size different from the size of the other coding unitsmerely corresponds to an example of determining a coding unit at apreset location by using the sizes of coding units, which are determinedbased on coordinates of samples, and thus various methods of determininga coding unit at a preset location by comparing the sizes of codingunits, which are determined based on coordinates of preset samples, maybe used.

However, locations of samples considered to determine locations ofcoding units are not limited to the above-described upper leftlocations, and information about arbitrary locations of samples includedin the coding units may be used.

According to an embodiment, the image decoding apparatus 100 may selecta coding unit at a preset location from among an odd number of codingunits determined by splitting the current coding unit, considering theshape of the current coding unit. For example, when the current codingunit has a non-square shape, a width of which is longer than a height,the image decoding apparatus 100 may determine the coding unit at thepreset location in a horizontal direction. That is, the image decodingapparatus 100 may determine one of coding units at different locationsin a horizontal direction and may put a restriction on the coding unit.When the current coding unit has a non-square shape, a height of whichis longer than a width, the image decoding apparatus 100 may determinethe coding unit at the preset location in a vertical direction. That is,the image decoding apparatus 100 may determine one of coding units atdifferent locations in a vertical direction and may put a restriction onthe coding unit.

According to an embodiment, the image decoding apparatus 100 may useinformation indicating respective locations of an even number of codingunits, to determine the coding unit at the preset location from amongthe even number of coding units. The image decoding apparatus 100 maydetermine an even number of coding units by splitting (binary splitting)the current coding unit, and may determine the coding unit at the presetlocation by using the information about the locations of the even numberof coding units. An operation related thereto may correspond to theoperation of determining a coding unit at a preset location (e.g., acenter location) from among an odd number of coding units, which hasbeen described in detail above in relation to FIG. 6 , and thus detaileddescriptions thereof are not provided here.

According to an embodiment, when a non-square current coding unit issplit into a plurality of coding units, preset information about acoding unit at a preset location may be used in a splitting operation todetermine the coding unit at the preset location from among theplurality of coding units. For example, the image decoding apparatus 100may use at least one of block shape information and split shape modeinformation, which is stored in a sample included in a middle codingunit, in a splitting operation to determine a coding unit at a centerlocation from among the plurality of coding units determined bysplitting the current coding unit.

Referring to FIG. 6 , the image decoding apparatus 100 may split thecurrent coding unit 600 into the plurality of coding units 620 a, 620 b,and 620 c based on the split shape mode information, and may determinethe coding unit 620 b at a center location from among the plurality ofthe coding units 620 a, 620 b, and 620 c. Furthermore, the imagedecoding apparatus 100 may determine the coding unit 620 b at the centerlocation, in consideration of a location from which the split shape modeinformation is obtained. That is, the split shape mode information ofthe current coding unit 600 may be obtained from the sample 640 at acenter location of the current coding unit 600 and, when the currentcoding unit 600 is split into the plurality of coding units 620 a, 620b, and 620 c based on the split shape mode information, the coding unit620 b including the sample 640 may be determined as the coding unit atthe center location. However, information used to determine the codingunit at the center location is not limited to the split shape modeinformation, and various types of information may be used to determinethe coding unit at the center location.

According to an embodiment, preset information for identifying thecoding unit at the preset location may be obtained from a preset sampleincluded in a coding unit to be determined. Referring to FIG. 6 , theimage decoding apparatus 100 may use the split shape mode information,which is obtained from a sample at a preset location in the currentcoding unit 600 (e.g., a sample at a center location of the currentcoding unit 600) to determine a coding unit at a preset location fromamong the plurality of the coding units 620 a, 620 b, and 620 cdetermined by splitting the current coding unit 600 (e.g., a coding unitat a center location from among a plurality of split coding units). Thatis, the image decoding apparatus 100 may determine the sample at thepreset location by considering a block shape of the current coding unit600, may determine the coding unit 620 b including a sample, from whichpreset information (e.g., the split shape mode information) can beobtained, from among the plurality of coding units 620 a, 620 b, and 620c determined by splitting the current coding unit 600, and may put apreset restriction on the coding unit 620 b. Referring to FIG. 6 ,according to an embodiment, the image decoding apparatus 100 maydetermine the sample 640 at the center location of the current codingunit 600 as the sample from which the preset information may beobtained, and may put a preset restriction on the coding unit 620 bincluding the sample 640, in a decoding operation. However, the locationof the sample from which the preset information can be obtained is notlimited to the above-described location, and may include arbitrarylocations of samples included in the coding unit 620 b to be determinedfor a restriction.

According to an embodiment, the location of the sample from which thepreset information may be obtained may be determined based on the shapeof the current coding unit 600. According to an embodiment, the blockshape information may indicate whether the current coding unit has asquare or non-square shape, and the location of the sample from whichthe preset information may be obtained may be determined based on theshape. For example, the image decoding apparatus 100 may determine asample located on a boundary for splitting at least one of a width andheight of the current coding unit in half, as the sample from which thepreset information can be obtained, by using at least one of informationabout the width of the current coding unit and information about theheight of the current coding unit. As another example, when the blockshape information of the current coding unit indicates a non-squareshape, the image decoding apparatus 100 may determine one of samplesadjacent to a boundary for splitting a long side of the current codingunit in half, as the sample from which the preset information can beobtained.

According to an embodiment, when the current coding unit is split into aplurality of coding units, the image decoding apparatus 100 may use thesplit shape mode information to determine a coding unit at a presetlocation from among the plurality of coding units. According to anembodiment, the image decoding apparatus 100 may obtain the split shapemode information from a sample at a preset location in a coding unit,and may split the plurality of coding units, which are generated bysplitting the current coding unit, by using the split shape modeinformation, which is obtained from the sample of the preset location ineach of the plurality of coding units. That is, a coding unit may berecursively split based on the split shape mode information, which isobtained from the sample at the preset location in each coding unit. Anoperation of recursively splitting a coding unit has been describedabove in relation to FIG. 5 , and thus detailed descriptions thereofwill not be provided here.

According to an embodiment, the image decoding apparatus 100 maydetermine one or more coding units by splitting the current coding unit,and may determine an order of decoding the one or more coding units,based on a preset block (e.g., the current coding unit).

FIG. 7 illustrates an order of processing a plurality of coding unitswhen the image decoding apparatus 100 determines the plurality of codingunits by splitting a current coding unit, according to an embodiment.

According to an embodiment, the image decoding apparatus 100 maydetermine second coding units 710 a and 710 b by splitting a firstcoding unit 700 in a vertical direction, may determine second codingunits 730 a and 730 b by splitting the first coding unit 700 in ahorizontal direction, or may determine second coding units 750 a, 750 b,750 c, and 750 d by splitting the first coding unit 700 in vertical andhorizontal directions, based on split shape mode information.

Referring to FIG. 7 , the image decoding apparatus 100 may determine toprocess the second coding units 710 a and 710 b, which are determined bysplitting the first coding unit 700 in a vertical direction, in ahorizontal direction order 710 c. The image decoding apparatus 100 maydetermine to process the second coding units 730 a and 730 b, which aredetermined by splitting the first coding unit 700 in a horizontaldirection, in a vertical direction order 730 c. The image decodingapparatus 100 may determine the second coding units 750 a, 750 b, 750 c,and 750 d, which are determined by splitting the first coding unit 700in vertical and horizontal directions, according to a preset order(e.g., a raster scan order or Z-scan order 750 e) by which coding unitsin a row are processed and then coding units in a next row areprocessed.

According to an embodiment, the image decoding apparatus 100 mayrecursively split coding units. Referring to FIG. 7 , the image decodingapparatus 100 may determine the plurality of coding units 710 a and 710b, 730 a and 730 b, or 750 a, 750 b, 750 c, and 750 d by splitting thefirst coding unit 700, and may recursively split each of the determinedplurality of coding units 710 a, 710 b, 730 a, 730 b, 750 a, 750 b, 750c, and 750 d. A splitting method of the plurality of coding units 710 aand 710 b, 730 a and 730 b, or 750 a, 750 b, 750 c, and 750 d maycorrespond to a splitting method of the first coding unit 700.Accordingly, each of the plurality of coding units 710 a and 710 b, 730a and 730 b, or 750 a, 750 b, 750 c, and 750 d may be independentlysplit into a plurality of coding units. Referring to FIG. 7 , the imagedecoding apparatus 100 may determine the second coding units 710 a and710 b by splitting the first coding unit 700 in a vertical direction,and may determine to independently split or to not split each of thesecond coding units 710 a and 710 b.

According to an embodiment, the image decoding apparatus 100 maydetermine third coding units 720 a and 720 b by splitting the leftsecond coding unit 710 a in a horizontal direction, and may not splitthe right second coding unit 710 b.

According to an embodiment, a processing order of coding units may bedetermined based on an operation of splitting a coding unit. In otherwords, a processing order of split coding units may be determined basedon a processing order of coding units immediately before being split.The image decoding apparatus 100 may determine a processing order of thethird coding units 720 a and 720 b determined by splitting the leftsecond coding unit 710 a, independently of the right second coding unit710 b. Because the third coding units 720 a and 720 b are determined bysplitting the left second coding unit 710 a in a horizontal direction,the third coding units 720 a and 720 b may be processed in a verticaldirection order 720 c. Because the left and right second coding units710 a and 710 b are processed in the horizontal direction order 710 c,the right second coding unit 710 b may be processed after the thirdcoding units 720 a and 720 b included in the left second coding unit 710a are processed in the vertical direction order 720 c. An operation ofdetermining a processing order of coding units based on a coding unitbefore being split is not limited to the above-described example, andvarious methods may be used to independently process coding units, whichare split and determined to various shapes, in a preset order.

FIG. 8 illustrates a process, performed by the image decoding apparatus100, of determining that a current coding unit is to be split into anodd number of coding units, when the coding units are not processable ina preset order, according to an embodiment.

According to an embodiment, the image decoding apparatus 100 maydetermine that the current coding unit is to be split into an odd numberof coding units, based on obtained split shape mode information.Referring to FIG. 8 , a square first coding unit 800 may be split intonon-square second coding units 810 a and 810 b, and the second codingunits 810 a and 810 b may be independently split into third coding units820 a and 820 b, and 820 c, 820 d, and 820 e. According to anembodiment, the image decoding apparatus 100 may determine the pluralityof third coding units 820 a and 820 b by splitting the left secondcoding unit 810 a in a horizontal direction, and may split the rightsecond coding unit 810 b into the odd number of third coding units 820c, 820 d, and 820 e.

According to an embodiment, the video decoding apparatus 100 maydetermine whether any coding unit is split into an odd number of codingunits, by determining whether the third coding units 820 a and 820 b,and 820 c, 820 d, and 820 e are processable in a preset order. Referringto FIG. 8 , the image decoding apparatus 100 may determine the thirdcoding units 820 a and 820 b, and 820 c, 820 d, and 820 e by recursivelysplitting the first coding unit 800. The image decoding apparatus 100may determine whether any of the first coding unit 800, the secondcoding units 810 a and 810 b, or the third coding units 820 a and 820 b,and 820 c, 820 d, and 820 e are split into an odd number of codingunits, based on at least one of the block shape information and thesplit shape mode information. For example, a coding unit located in theright from among the second coding units 810 a and 810 b may be splitinto an odd number of third coding units 820 c, 820 d, and 820 e. Aprocessing order of a plurality of coding units included in the firstcoding unit 800 may be a preset order (e.g., a Z-scan order 830), andthe image decoding apparatus 100 may determine whether the third codingunits 820 c, 820 d, and 820 e, which are determined by splitting theright second coding unit 810 b into an odd number of coding units,satisfy a condition for processing in the preset order.

According to an embodiment, the image decoding apparatus 100 maydetermine whether the third coding units 820 a and 820 b, and 820 c, 820d, and 820 e included in the first coding unit 800 satisfy the conditionfor processing in the preset order, and the condition relates to whetherat least one of a width and height of the second coding units 810 a and810 b is to be split in half along a boundary of the third coding units820 a and 820 b, and 820 c, 820 d, and 820 e. For example, the thirdcoding units 820 a and 820 b determined when the height of the leftsecond coding unit 810 a of the non-square shape is split in half maysatisfy the condition. It may be determined that the third coding units820 c, 820 d, and 820 e do not satisfy the condition because theboundaries of the third coding units 820 c, 820 d, and 820 e determinedwhen the right second coding unit 810 b is split into three coding unitsare unable to split the width or height of the right second coding unit810 b in half. When the condition is not satisfied as described above,the image decoding apparatus 100 may determine disconnection of a scanorder, and may determine that the right second coding unit 810 b is tobe split into an odd number of coding units, based on a result of thedetermination. According to an embodiment, when a coding unit is splitinto an odd number of coding units, the image decoding apparatus 100 mayput a preset restriction on a coding unit at a preset location fromamong the split coding units. The restriction or the preset location hasbeen described above in relation to various embodiments, and thusdetailed descriptions thereof will not be provided herein.

FIG. 9 illustrates a process, performed by the image decoding apparatus100, of determining at least one coding unit by splitting a first codingunit 900, according to an embodiment.

According to an embodiment, the image decoding apparatus 100 may splitthe first coding unit 900, based on split shape mode information, whichis obtained through the bitstream obtainer 110. The square first codingunit 900 may be split into four square coding units, or may be splitinto a plurality of non-square coding units. For example, referring toFIG. 9 , when the first coding unit 900 has a square shape and the splitshape mode information indicates to split the first coding unit 900 intonon-square coding units, the image decoding apparatus 100 may split thefirst coding unit 900 into a plurality of non-square coding units. Indetail, when the split shape mode information indicates to determine anodd number of coding units by splitting the first coding unit 900 in ahorizontal direction or a vertical direction, the image decodingapparatus 100 may split the square first coding unit 900 into an oddnumber of coding units, e.g., second coding units 910 a, 910 b, and 910c determined by splitting the square first coding unit 900 in a verticaldirection or second coding units 920 a, 920 b, and 920 c determined bysplitting the square first coding unit 900 in a horizontal direction.

According to an embodiment, the image decoding apparatus 100 maydetermine whether the second coding units 910 a, 910 b, 910 c, 920 a,920 b, and 920 c included in the first coding unit 900 satisfy acondition for processing in a preset order, and the condition relates towhether at least one of a width and height of the first coding unit 900is to be split in half along a boundary of the second coding units 910a, 910 b, 910 c, 920 a, 920 b, and 920 c. Referring to FIG. 9 , becauseboundaries of the second coding units 910 a, 910 b, and 910 c determinedby splitting the square first coding unit 900 in a vertical direction donot split the width of the first coding unit 900 in half, it may bedetermined that the first coding unit 900 does not satisfy the conditionfor processing in the preset order. Also, because boundaries of thesecond coding units 920 a, 920 b, and 920 c determined by splitting thesquare first coding unit 900 in a horizontal direction do not split theheight of the first coding unit 900 in half, it may be determined thatthe first coding unit 900 does not satisfy the condition for processingin the preset order. When the condition is not satisfied as describedabove, the image decoding apparatus 100 may decide disconnection of ascan order, and may determine that the first coding unit 900 is to besplit into an odd number of coding units, based on a result of thedecision. According to an embodiment, when a coding unit is split intoan odd number of coding units, the image decoding apparatus 100 may puta preset restriction on a coding unit at a preset location from amongthe split coding units. The restriction or the preset location has beendescribed above in relation to various embodiments, and thus detaileddescriptions thereof will not be provided herein.

According to an embodiment, the image decoding apparatus 100 maydetermine various-shaped coding units by splitting a first coding unit.

Referring to FIG. 9 , the image decoding apparatus 100 may split thesquare first coding unit 900 or a non-square first coding unit 930 or950 into various-shaped coding units.

FIG. 10 illustrates that a shape into which a second coding unit issplittable is restricted when the second coding unit having a non-squareshape, which is determined when the image decoding apparatus 100 splitsa first coding unit 1000, satisfies a preset condition, according to anembodiment.

According to an embodiment, the image decoding apparatus 100 maydetermine to split the square first coding unit 1000 into non-squaresecond coding units 1010 a, and 1010 b or 1020 a and 1020 b, based onsplit shape mode information, which is obtained by the bitstreamobtainer 110. The second coding units 1010 a and 1010 b, or 1020 a and1020 b may be independently split. As such, the image decoding apparatus100 may determine to split or to not split each of the second codingunits 1010 a and 1010 b, or 1020 a and 1020 b into a plurality of codingunits, based on the split shape mode information of each of the secondcoding units 1010 a and 1010 b, or 1020 a and 1020 b. According to anembodiment, the image decoding apparatus 100 may determine third codingunits 1012 a and 1012 b by splitting the non-square left second codingunit 1010 a, which is determined by splitting the first coding unit 1000in a vertical direction, in a horizontal direction. However, when theleft second coding unit 1010 a is split in a horizontal direction, theimage decoding apparatus 100 may restrict the right second coding unit1010 b to not be split in a horizontal direction in which the leftsecond coding unit 1010 a is split. When third coding units 1014 a and1014 b are determined by splitting the right second coding unit 1010 bin a same direction, because the left and right second coding units 1010a and 1010 b are independently split in a horizontal direction, thethird coding units 1012 a and 1012 b, or 1014 a and 1014 b may bedetermined. However, this case serves equally as a case in which theimage decoding apparatus 100 splits the first coding unit 1000 into foursquare second coding units 1030 a, 1030 b, 1030 c, and 1030 d, based onthe split shape mode information, and may be inefficient in terms ofimage decoding.

According to an embodiment, the image decoding apparatus 100 maydetermine third coding units 1022 a and 1022 b, or 1024 a and 1024 b bysplitting the non-square second coding unit 1020 a or 1020 b, which isdetermined by splitting the first coding unit 1000 in a horizontaldirection, in a vertical direction. However, when a second coding unit(e.g., the upper second coding unit 1020 a) is split in a verticaldirection, for the above-described reason, the image decoding apparatus100 may restrict the other second coding unit (e.g., the lower secondcoding unit 1020 b) to not be split in a vertical direction in which theupper second coding unit 1020 a is split.

FIG. 11 illustrates a process, performed by the image decoding apparatus100, of splitting a square coding unit when split shape mode informationindicates that the square coding unit is to not be split into foursquare coding units, according to an embodiment.

According to an embodiment, the image decoding apparatus 100 maydetermine second coding units 1110 a and 1110 b, or 1120 a and 1120 b,etc. by splitting a first coding unit 1100, based on split shape modeinformation. The split shape mode information may include informationabout various methods of splitting a coding unit but, the informationabout various splitting methods may not include information forsplitting a coding unit into four square coding units. According to suchsplit shape mode information, the image decoding apparatus 100 may notsplit the square first coding unit 1100 into four square second codingunits 1130 a, 1130 b, 1130 c, and 1130 d. The image decoding apparatus100 may determine the non-square second coding units 1110 a and 1110 b,or 1120 a and 1120 b, etc., based on the split shape mode information.

According to an embodiment, the image decoding apparatus 100 mayindependently split the non-square second coding units 1110 a and 1110b, or 1120 a and 1120 b, etc. Each of the second coding units 1110 a and1110 b, or 1120 a and 1120 b, etc. may be recursively split in a presetorder, and this splitting method may correspond to a method of splittingthe first coding unit 1100, based on the split shape mode information.

For example, the image decoding apparatus 100 may determine square thirdcoding units 1112 a and 1112 b by splitting the left second coding unit1110 a in a horizontal direction, and may determine square third codingunits 1114 a and 1114 b by splitting the right second coding unit 1110 bin a horizontal direction. Furthermore, the image decoding apparatus 100may determine square third coding units 1116 a, 1116 b, 1116 c, and 1116d by splitting both of the left and right second coding units 1110 a and1110 b in a horizontal direction. In this case, coding units having thesame shape as the four square second coding units 1130 a, 1130 b, 1130c, and 1130 d split from the first coding unit 1100 may be determined.

As another example, the image decoding apparatus 100 may determinesquare third coding units 1122 a and 1122 b by splitting the uppersecond coding unit 1120 a in a vertical direction, and may determinesquare third coding units 1124 a and 1124 b by splitting the lowersecond coding unit 1120 b in a vertical direction. Furthermore, theimage decoding apparatus 100 may determine square third coding units1126 a, 1126 b, 1126 c, and 1126 d by splitting both the upper and lowersecond coding units 1120 a and 1120 b in a vertical direction. In thiscase, coding units having the same shape as the four square secondcoding units 1130 a, 1130 b, 1130 c, and 1130 d split from the firstcoding unit 1100 may be determined.

FIG. 12 illustrates that a processing order between a plurality ofcoding units may be changed depending on a process of splitting a codingunit, according to an embodiment.

According to an embodiment, the image decoding apparatus 100 may split afirst coding unit 1200, based on split shape mode information. When ablock shape indicates a square shape and the split shape modeinformation indicates to split the first coding unit 1200 in at leastone of horizontal and vertical directions, the image decoding apparatus100 may determine second coding units 1210 a and 1210 b, or 1220 a and1220 b, etc. by splitting the first coding unit 1200. Referring to FIG.12 , the non-square second coding units 1210 a and 1210 b, or 1220 a and1220 b determined by splitting the first coding unit 1200 in only ahorizontal direction or vertical direction may be independently splitbased on the split shape mode information of each coding unit. Forexample, the image decoding apparatus 100 may determine third codingunits 1216 a, 1216 b, 1216 c, and 1216 d by splitting the second codingunits 1210 a and 1210 b, which are generated by splitting the firstcoding unit 1200 in a vertical direction, in a horizontal direction, andmay determine third coding units 1226 a, 1226 b, 1226 c, and 1226 d bysplitting the second coding units 1220 a and 1220 b, which are generatedby splitting the first coding unit 1200 in a horizontal direction, in avertical direction. An operation of splitting the second coding units1210 a and 1210 b, or 1220 a and 1220 b has been described above inrelation to FIG. 11 , and thus detailed descriptions thereof will not beprovided herein.

According to an embodiment, the image decoding apparatus 100 may processcoding units in a preset order. An operation of processing coding unitsin a preset order has been described above in relation to FIG. 7 , andthus detailed descriptions thereof will not be provided herein.Referring to FIG. 12 , the image decoding apparatus 100 may determinefour square third coding units 1216 a, 1216 b, 1216 c, and 1216 d, and1226 a, 1226 b, 1226 c, and 1226 d by splitting the square first codingunit 1200. According to an embodiment, the image decoding apparatus 100may determine processing orders of the third coding units 1216 a, 1216b, 1216 c, and 1216 d, and 1226 a, 1226 b, 1226 c, and 1226 d based on asplit shape by which the first coding unit 1200 is split.

According to an embodiment, the image decoding apparatus 100 maydetermine the third coding units 1216 a, 1216 b, 1216 c, and 1216 d bysplitting the second coding units 1210 a and 1210 b generated bysplitting the first coding unit 1200 in a vertical direction, in ahorizontal direction, and may process the third coding units 1216 a,1216 b, 1216 c, and 1216 d in a processing order 1217 for initiallyprocessing the third coding units 1216 a and 1216 c, which are includedin the left second coding unit 1210 a, in a vertical direction and thenprocessing the third coding unit 1216 b and 1216 d, which are includedin the right second coding unit 1210 b, in a vertical direction.

According to an embodiment, the image decoding apparatus 100 maydetermine the third coding units 1226 a, 1226 b, 1226 c, and 1226 d bysplitting the second coding units 1220 a and 1220 b generated bysplitting the first coding unit 1200 in a horizontal direction, in avertical direction, and may process the third coding units 1226 a, 1226b, 1226 c, and 1226 d in a processing order 1227 for initiallyprocessing the third coding units 1226 a and 1226 b, which are includedin the upper second coding unit 1220 a, in a horizontal direction andthen processing the third coding unit 1226 c and 1226 d, which areincluded in the lower second coding unit 1220 b, in a horizontaldirection.

Referring to FIG. 12 , the square third coding units 1216 a, 1216 b,1216 c, and 1216 d, and 1226 a, 1226 b, 1226 c, and 1226 d may bedetermined by splitting the second coding units 1210 a and 1210 b, and1220 a and 1220 b, respectively. Although the second coding units 1210 aand 1210 b are determined by splitting the first coding unit 1200 in avertical direction differently from the second coding units 1220 a and1220 b which are determined by splitting the first coding unit 1200 in ahorizontal direction, the third coding units 1216 a, 1216 b, 1216 c, and1216 d, and 1226 a, 1226 b, 1226 c, and 1226 d split therefromeventually show same-shaped coding units split from the first codingunit 1200. As such, by recursively splitting a coding unit in differentmanners based on the split shape mode information, the image decodingapparatus 100 may process a plurality of coding units in differentorders even when the coding units are eventually determined to be thesame shape.

FIG. 13 illustrates a process of determining a depth of a coding unit asa shape and a size of the coding unit change, when the coding unit isrecursively split such that a plurality of coding units are determined,according to an embodiment.

According to an embodiment, the image decoding apparatus 100 maydetermine the depth of the coding unit, based on a preset criterion. Forexample, the preset criterion may be the length of a long side of thecoding unit. When the length of a long side of a coding unit beforebeing split is 2n times (n>0) the length of a long side of a splitcurrent coding unit, the image decoding apparatus 100 may determine thata depth of the current coding unit is increased from a depth of thecoding unit before being split, by n. In the following descriptions, acoding unit having an increased depth is expressed as a coding unit of alower depth.

Referring to FIG. 13 , according to an embodiment, the image decodingapparatus 100 may determine a second coding unit 1302 and a third codingunit 1304 of lower depths by splitting a square first coding unit 1300based on block shape information indicating a square shape (e.g., theblock shape information may be expressed as ‘0: SQUARE’). Assuming thatthe size of the square first coding unit 1300 is 2N×2N, the secondcoding unit 1302 determined by splitting a width and height of the firstcoding unit 1300 in ½ may have a size of N×N. Furthermore, the thirdcoding unit 1304 determined by splitting a width and height of thesecond coding unit 1302 in ½ may have a size of N/2×N/2. In this case, awidth and height of the third coding unit 1304 are ¼ times those of thefirst coding unit 1300. When a depth of the first coding unit 1300 is D,a depth of the second coding unit 1302, the width and height of whichare ½ times those of the first coding unit 1300, may be D+1, and a depthof the third coding unit 1304, the width and height of which are ¼ timesthose of the first coding unit 1300, may be D+2.

According to an embodiment, the image decoding apparatus 100 maydetermine a second coding unit 1312 or 1322 and a third coding unit 1314or 1324 of lower depths by splitting a non-square first coding unit 1310or 1320 based on block shape information indicating a non-square shape(e.g., the block shape information may be expressed as ‘1: NS_VER’indicating a non-square shape, a height of which is longer than a width,or as ‘2: NS_HOR’ indicating a non-square shape, a width of which islonger than a height).

The image decoding apparatus 100 may determine a second coding unit1302, 1312, or 1322 by splitting at least one of a width and height ofthe first coding unit 1310 having a size of N×2N. That is, the imagedecoding apparatus 100 may determine the second coding unit 1302 havinga size of N×N or the second coding unit 1322 having a size of N×N/2 bysplitting the first coding unit 1310 in a horizontal direction, or maydetermine the second coding unit 1312 having a size of N/2×N bysplitting the first coding unit 1310 in horizontal and verticaldirections.

According to an embodiment, the image decoding apparatus 100 maydetermine the second coding unit 1302, 1312, or 1322 by splitting atleast one of a width and height of the first coding unit 1320 having asize of 2N×N. That is, the image decoding apparatus 100 may determinethe second coding unit 1302 having a size of N×N or the second codingunit 1312 having a size of N/2×N by splitting the first coding unit 1320in a vertical direction, or may determine the second coding unit 1322having a size of N×N/2 by splitting the first coding unit 1320 inhorizontal and vertical directions.

According to an embodiment, the image decoding apparatus 100 maydetermine a third coding unit 1304, 1314, or 1324 by splitting at leastone of a width and height of the second coding unit 1302 having a sizeof N×N. That is, the image decoding apparatus 100 may determine thethird coding unit 1304 having a size of N/2×N/2, the third coding unit1314 having a size of N/4×N/2, or the third coding unit 1324 having asize of N/2×N/4 by splitting the second coding unit 1302 in vertical andhorizontal directions.

According to an embodiment, the image decoding apparatus 100 maydetermine the third coding unit 1304, 1314, or 1324 by splitting atleast one of a width and height of the second coding unit 1312 having asize of N/2×N. That is, the image decoding apparatus 100 may determinethe third coding unit 1304 having a size of N/2×N/2 or the third codingunit 1324 having a size of N/2×N/4 by splitting the second coding unit1312 in a horizontal direction, or may determine the third coding unit1314 having a size of N/4×N/2 by splitting the second coding unit 1312in vertical and horizontal directions.

According to an embodiment, the image decoding apparatus 100 maydetermine the third coding unit 1304, 1314, or 1324 by splitting atleast one of a width and height of the second coding unit 1322 having asize of N×N/2. That is, the image decoding apparatus 100 may determinethe third coding unit 1304 having a size of N/2×N/2 or the third codingunit 1314 having a size of N/4×N/2 by splitting the second coding unit1322 in a vertical direction, or may determine the third coding unit1324 having a size of N/2×N/4 by splitting the second coding unit 1322in vertical and horizontal directions.

According to an embodiment, the image decoding apparatus 100 may splitthe square coding unit 1300, 1302, or 1304 in a horizontal or verticaldirection. For example, the image decoding apparatus 100 may determinethe first coding unit 1310 having a size of N×2N by splitting the firstcoding unit 1300 having a size of 2N×2N in a vertical direction, or maydetermine the first coding unit 1320 having a size of 2N×N by splittingthe first coding unit 1300 in a horizontal direction. According to anembodiment, when a depth is determined based on the length of thelongest side of a coding unit, a depth of a coding unit determined bysplitting the first coding unit 1300 having a size of 2N×2N in ahorizontal or vertical direction may be the same as the depth of thefirst coding unit 1300.

According to an embodiment, a width and height of the third coding unit1314 or 1324 may be ¼ times those of the first coding unit 1310 or 1320.When a depth of the first coding unit 1310 or 1320 is D, a depth of thesecond coding unit 1312 or 1322, the width and height of which are ½times those of the first coding unit 1310 or 1320, may be D+1, and adepth of the third coding unit 1314 or 1324, the width and height ofwhich are ¼ times those of the first coding unit 1310 or 1320, may beD+2.

FIG. 14 illustrates depths that are determinable based on shapes andsizes of coding units, and part indexes (PIDs) that are fordistinguishing the coding units, according to an embodiment.

According to an embodiment, the image decoding apparatus 100 maydetermine various-shape second coding units by splitting a square firstcoding unit 1400. Referring to FIG. 14 , the image decoding apparatus100 may determine second coding units 1402 a and 1402 b, 1404 a and 1404b, and 1406 a, 1406 b, 1406 c, and 1406 d by splitting the first codingunit 1400 in at least one of vertical and horizontal directions based onsplit shape mode information. That is, the image decoding apparatus 100may determine the second coding units 1402 a and 1402 b, 1404 a and 1404b, and 1406 a, 1406 b, 1406 c, and 1406 d, based on the split shape modeinformation of the first coding unit 1400.

According to an embodiment, depths of the second coding units 1402 a and1402 b, 1404 a and 1404 b, and 1406 a, 1406 b, 1406 c, and 1406 d thatare determined based on the split shape mode information of the squarefirst coding unit 1400 may be determined based on the length of a longside thereof. For example, because the length of a side of the squarefirst coding unit 1400 equals the length of a long side of thenon-square second coding units 1402 a and 1402 b, and 1404 a and 1404 b,the first coding unit 1400 and the non-square second coding units 1402 aand 1402 b, and 1404 a and 1404 b may have the same depth, e.g., D.However, when the image decoding apparatus 100 splits the first codingunit 1400 into the four square second coding units 1406 a, 1406 b, 1406c, and 1406 d based on the split shape mode information, because thelength of a side of the square second coding units 1406 a, 1406 b, 1406c, and 1406 d is ½ times the length of a side of the first coding unit1400, a depth of the second coding units 1406 a, 1406 b, 1406 c, and1406 d may be D+1 which is deeper than the depth D of the first codingunit 1400 by 1.

According to an embodiment, the image decoding apparatus 100 maydetermine a plurality of second coding units 1412 a and 1412 b, and 1414a, 1414 b, and 1414 c by splitting a first coding unit 1410, a height ofwhich is longer than a width, in a horizontal direction based on thesplit shape mode information. According to an embodiment, the imagedecoding apparatus 100 may determine a plurality of second coding units1422 a and 1422 b, and 1424 a, 1424 b, and 1424 c by splitting a firstcoding unit 1420, a width of which is longer than a height, in avertical direction based on the split shape mode information.

According to an embodiment, a depth of the second coding units 1412 aand 1412 b, and 1414 a, 1414 b, and 1414 c, or 1422 a and 1422 b, and1424 a, 1424 b, and 1424 c, which are determined based on the splitshape mode information of the non-square first coding unit 1410 or 1420,may be determined based on the length of a long side thereof. Forexample, because the length of a side of the square second coding units1412 a and 1412 b is ½ times the length of a long side of the firstcoding unit 1410 having a non-square shape, a height of which is longerthan a width, a depth of the square second coding units 1412 a and 1412b is D+1 which is deeper than the depth D of the non-square first codingunit 1410 by 1.

Furthermore, the image decoding apparatus 100 may split the non-squarefirst coding unit 1410 into an odd number of second coding units 1414 a,1414 b, and 1414 c based on the split shape mode information. The oddnumber of second coding units 1414 a, 1414 b, and 1414 c may include thenon-square second coding units 1414 a and 1414 c and the square secondcoding unit 1414 b. In this case, because the length of a long side ofthe non-square second coding units 1414 a and 1414 c and the length of aside of the square second coding unit 1414 b are ½ times the length of along side of the first coding unit 1410, a depth of the second codingunits 1414 a, 1414 b, and 1414 c may be D+1 which is deeper than thedepth D of the non-square first coding unit 1410 by 1. The imagedecoding apparatus 100 may determine depths of coding units split fromthe first coding unit 1420 having a non-square shape, a width of whichis longer than a height, by using the above-described method ofdetermining depths of coding units split from the first coding unit1410.

According to an embodiment, the image decoding apparatus 100 maydetermine PIDs for identifying split coding units, based on a size ratiobetween the coding units when an odd number of split coding units do nothave equal sizes. Referring to FIG. 14 , a coding unit 1414 b of acenter location among an odd number of split coding units 1414 a, 1414b, and 1414 c may have a width equal to that of the other coding units1414 a and 1414 c and a height which is two times that of the othercoding units 1414 a and 1414 c. That is, in this case, the coding unit1414 b at the center location may include two of the other coding unit1414 a or 1414 c. Therefore, when a PID of the coding unit 1414 b at thecenter location is 1 based on a scan order, a PID of the coding unit1414 c located next to the coding unit 1414 b may be increased by 2 andthus may be 3. That is, discontinuity in PID values may be present.According to an embodiment, the image decoding apparatus 100 maydetermine whether an odd number of split coding units do not have equalsizes, based on whether discontinuity is present in PIDs for identifyingthe split coding units.

According to an embodiment, the image decoding apparatus 100 maydetermine whether to use a specific splitting method, based on PIDvalues for identifying a plurality of coding units determined bysplitting a current coding unit. Referring to FIG. 14 , the imagedecoding apparatus 100 may determine an even number of coding units 1412a and 1412 b or an odd number of coding units 1414 a, 1414 b, and 1414 cby splitting the first coding unit 1410 having a rectangular shape, aheight of which is longer than a width. The image decoding apparatus 100may use PIDs indicating respective coding units so as to identify therespective coding units. According to an embodiment, the PID may beobtained from a sample at a preset location of each coding unit (e.g.,an upper-left sample).

According to an embodiment, the image decoding apparatus 100 maydetermine a coding unit at a preset location from among the split codingunits, by using the PIDs for distinguishing the coding units. Accordingto an embodiment, when the split shape mode information of the firstcoding unit 1410 having a rectangular shape, a height of which is longerthan a width, indicates to split a coding unit into three coding units,the image decoding apparatus 100 may split the first coding unit 1410into three coding units 1414 a, 1414 b, and 1414 c. The image decodingapparatus 100 may assign a PID to each of the three coding units 1414 a,1414 b, and 1414 c. The image decoding apparatus 100 may compare PIDs ofan odd number of split coding units to determine a coding unit at acenter location from among the coding units. The image decodingapparatus 100 may determine the coding unit 1414 b having a PIDcorresponding to a middle value among the PIDs of the coding units, asthe coding unit at the center location from among the coding unitsdetermined by splitting the first coding unit 1410. According to anembodiment, the image decoding apparatus 100 may determine PIDs fordistinguishing split coding units, based on a size ratio between thecoding units when the split coding units do not have equal sizes.Referring to FIG. 14 , the coding unit 1414 b generated by splitting thefirst coding unit 1410 may have a width equal to that of the othercoding units 1414 a and 1414 c and a height which is two times that ofthe other coding units 1414 a and 1414 c. In this case, when the PID ofthe coding unit 1414 b at the center location is 1, the PID of thecoding unit 1414 c located next to the coding unit 1414 b may beincreased by 2 and thus may be 3. When the PID is not uniformlyincreased as described above, the image decoding apparatus 100 maydetermine that a coding unit is split into a plurality of coding unitsincluding a coding unit having a size different from that of the othercoding units. According to an embodiment, when the split shape modeinformation indicates to split a coding unit into an odd number ofcoding units, the image decoding apparatus 100 may split a currentcoding unit in such a manner that a coding unit of a preset locationamong an odd number of coding units (e.g., a coding unit of a centerlocation) has a size different from that of the other coding units. Inthis case, the image decoding apparatus 100 may determine the codingunit of the center location, which has a different size, by using PIDsof the coding units. However, the PIDs and the size or location of thecoding unit of the preset location are not limited to theabove-described examples, and various PIDs and various locations andsizes of coding units may be used.

According to an embodiment, the image decoding apparatus 100 may use apreset data unit where a coding unit starts to be recursively split.

FIG. 15 illustrates that a plurality of coding units are determinedbased on a plurality of preset data units included in a picture,according to an embodiment.

According to an embodiment, a preset data unit may be defined as a dataunit where a coding unit starts to be recursively split by using splitshape mode information. That is, the preset data unit may correspond toa coding unit of an uppermost depth, which is used to determine aplurality of coding units split from a current picture. In the followingdescriptions, for convenience of explanation, the preset data unit isreferred to as a reference data unit.

According to an embodiment, the reference data unit may have a presetsize and a preset shape. According to an embodiment, a reference codingunit may include M×N samples. Herein, M and N may be equal to eachother, and may be integers expressed as powers of 2. That is, thereference data unit may have a square or non-square shape, and may besplit into an integer number of coding units.

According to an embodiment, the image decoding apparatus 100 may splitthe current picture into a plurality of reference data units. Accordingto an embodiment, the image decoding apparatus 100 may split theplurality of reference data units, which are split from the currentpicture, by using the split shape mode information of each referencedata unit. The operation of splitting the reference data unit maycorrespond to a splitting operation using a quadtree structure.

According to an embodiment, the image decoding apparatus 100 maypredetermine the minimum size allowed for the reference data unitsincluded in the current picture. Accordingly, the image decodingapparatus 100 may determine various reference data units having sizesequal to or greater than the minimum size, and may determine one or morecoding units by using the split shape mode information with reference tothe determined reference data unit.

Referring to FIG. 15 , the image decoding apparatus 100 may use a squarereference coding unit 1500 or a non-square reference coding unit 1502.According to an embodiment, the shape and size of reference coding unitsmay be determined based on various data units capable of including oneor more reference coding units (e.g., sequences, pictures, slices, slicesegments, tiles, tile groups, largest coding units, or the like).

According to an embodiment, the bitstream obtainer 110 of the imagedecoding apparatus 100 may obtain, from a bitstream, at least one ofreference coding unit shape information and reference coding unit sizeinformation with respect to each of the various data units. An operationof splitting the square reference coding unit 1500 into one or morecoding units has been described above in relation to the operation ofsplitting the current coding unit 300 of FIG. 3 , and an operation ofsplitting the non-square reference coding unit 1502 into one or morecoding units has been described above in relation to the operation ofsplitting the current coding unit 400 or 450 of FIG. 4 . Thus, detaileddescriptions thereof will not be provided herein.

According to an embodiment, the image decoding apparatus 100 may use aPID for identifying the size and shape of reference coding units, todetermine the size and shape of reference coding units according to somedata units predetermined based on a preset condition. That is, thebitstream obtainer 110 may obtain, from the bitstream, only the PID foridentifying the size and shape of reference coding units with respect toeach slice, slice segment, tile, tile group, or largest coding unitwhich is a data unit satisfying a preset condition (e.g., a data unithaving a size equal to or smaller than a slice) among the various dataunits (e.g., sequences, pictures, slices, slice segments, tiles, tilegroups, largest coding units, or the like). The image decoding apparatus100 may determine the size and shape of reference data units withrespect to each data unit, which satisfies the preset condition, byusing the PID. When the reference coding unit shape information and thereference coding unit size information are obtained and used from thebitstream according to each data unit having a relatively small size,efficiency of using the bitstream may not be high, and therefore, onlythe PID may be obtained and used instead of directly obtaining thereference coding unit shape information and the reference coding unitsize information. In this case, at least one of the size and shape ofreference coding units corresponding to the PID for identifying the sizeand shape of reference coding units may be predetermined. That is, theimage decoding apparatus 100 may determine at least one of the size andshape of reference coding units included in a data unit serving as aunit for obtaining the PID, by selecting the predetermined at least oneof the size and shape of reference coding units based on the PID.

According to an embodiment, the image decoding apparatus 100 may use oneor more reference coding units included in a largest coding unit. Thatis, a largest coding unit split from a picture may include one or morereference coding units, and coding units may be determined byrecursively splitting each reference coding unit. According to anembodiment, at least one of a width and height of the largest codingunit may be integer times at least one of the width and height of thereference coding units. According to an embodiment, the size ofreference coding units may be obtained by splitting the largest codingunit n times based on a quadtree structure. That is, the image decodingapparatus 100 may determine the reference coding units by splitting thelargest coding unit n times based on a quadtree structure, and may splitthe reference coding unit based on at least one of the block shapeinformation and the split shape mode information according to variousembodiments.

FIG. 16 illustrates a processing block used as criterion for determiningan order of determining reference coding units included in a picture1600, according to an embodiment.

According to an embodiment, the image decoding apparatus 100 maydetermine one or more processing blocks splitting a picture. Aprocessing block may be a data unit including one or more referencecoding units splitting an image, and one or more reference coding unitsincluded in a processing block may be determined according to a specificorder. That is, an order of determining one or more reference codingunits for each processing block may correspond to one of various typesof orders for determining reference coding units, and differentprocessing blocks may have different orders of determining referencecoding units. An order of determining reference coding units for eachprocessing block may be one of various orders, e.g., raster scan,Z-scan, N-scan, up-right diagonal scan, horizontal scan, and verticalscan, but is not limited to the above-mentioned scan orders.

According to an embodiment, the image decoding apparatus 100 may obtainprocessing block size information to determine a size of at least oneprocessing block included in an image. The image decoding apparatus 100may obtain the processing block size information from a bitstream todetermine the size of at least one processing block included in theimage. The size of the processing block may be a preset size of a dataunit, indicated by the processing block size information.

According to an embodiment, the bitstream obtainer 110 of the imagedecoding apparatus 100 may obtain processing block size information froma bitstream for each specific data unit. For example, the processingblock size information may be obtained from the bitstream in a dataunit, such as an image, a sequence, a picture, a slice, a slice segment,a tile, a tile group, etc. That is, the bitstream obtainer 110 mayobtain processing block size information from a bitstream for each ofthe various data units, and the image decoding apparatus 100 maydetermine a size of at least one processing block splitting a picture byusing the obtained processing block size information, wherein the sizeof the processing block may be an integer multiple of a reference codingunit.

According to an embodiment, the image decoding apparatus 100 maydetermine a size of processing blocks 1602 and 1612 included in thepicture 1600. For example, the image decoding apparatus 100 maydetermine a size of a processing block based on processing block sizeinformation obtained from a bitstream. Referring to FIG. 16 , accordingto an embodiment, the image decoding apparatus 100 may determine ahorizontal size of the processing blocks 1602 and 1612 to be four timesthat of a reference coding unit, and may determine a vertical size ofthe processing blocks 1602 and 1612 to be four times that of thereference coding unit. The image decoding apparatus 100 may determine anorder of determining one or more reference coding units in one or moreprocessing blocks.

According to an embodiment, the image decoding apparatus 100 maydetermine the processing blocks 1602 and 1612 included in the picture1600, based on the size of the processing block, and may determine anorder of determining one or more reference coding units included in theprocessing blocks 1602 and 1612. According to an embodiment, determininga reference coding unit may include determining a size of the referencecoding unit.

According to an embodiment, the image decoding apparatus 100 may obtain,from a bitstream, information about an order of determining one or morereference coding units included in one or more processing blocks, andmay determine an order of determining one or more reference coding unitsbased on the information about the determination order. The informationabout the determination order may be defined as an order or direction inwhich the reference coding units are determined in the processing block.That is, an order in which reference coding units are determined may beindependently determined for each processing block.

According to an embodiment, the image decoding apparatus 100 may obtain,from a bitstream, information about an order of determining referencecoding units for each specific data unit. For example, the bitstreamobtainer 110 may obtain information about an order of determiningreference coding units from a bitstream for each data unit, such as animage, a sequence, a picture, a slice, a slice segment, a tile, a tilegroup, a processing block, etc. Because information about an order ofdetermining reference coding units indicates an order of determiningreference coding units in a processing block, the information about thedetermination order may be obtained for each specific data unitincluding an integer number of processing blocks.

The image decoding apparatus 100 may determine one or more referencecoding units based on an order determined according to an embodiment.

According to an embodiment, the bitstream obtainer 110 may obtain, froma bitstream, information about an order of determining reference codingunits as information related to the processing blocks 1602 and 1612, andthe image decoding apparatus 100 may determine an order of determiningone or more reference coding units included in the processing blocks1602 and 1612, and determine one or more reference coding units includedin the picture 1600 according to the order of determining the referencecoding units. Referring to FIG. 16 , the image decoding apparatus 100may determine orders 1604 and 1614 of determining one or more referencecoding units with respect to the processing blocks 1602 and 1612,respectively. For example, when information about an order ofdetermining reference coding units is obtained for each processingblock, the processing blocks 1602 and 1612 may have different orders ofdetermining reference coding units. When the order 1604 of determiningreference coding units with respect to the processing block 1602 is anorder of raster scan, the reference coding units included in theprocessing block 1602 may be determined in the order of raster scan. Onthe contrary, when the order 1614 of determining reference coding unitswith respect to the other processing block 1612 is a reverse order ofraster scan, the reference coding units included in the processing block1612 may be determined in the reverse order of raster scan.

According to an embodiment, the image decoding apparatus 100 may decodethe one or more reference coding units. The image decoding apparatus 100may decode an image based on the reference coding units determinedthrough the above-described embodiment. A method of decoding referencecoding units may include various image decoding methods.

According to an embodiment, the image decoding apparatus 100 may obtainblock shape information indicating the shape of a current coding unit orsplit shape mode information indicating a splitting method of thecurrent coding unit, from the bitstream, and may use the obtainedinformation. The split shape mode information may be included in thebitstream related to various data units. For example, the image decodingapparatus 100 may use the split shape mode information included in asequence parameter set, a picture parameter set, a video parameter set,a slice header, a slice segment header, a tile header, or a tile groupheader. Furthermore, the image decoding apparatus 100 may obtain, fromthe bitstream, a syntax element corresponding to the block shapeinformation or the split shape mode information according to eachlargest coding unit, each reference coding unit, or each processingblock, and may use the obtained syntax element.

Hereinafter, a method of determining a split rule, according to anembodiment of the disclosure will be described in detail.

The image decoding apparatus 100 may determine a split rule of an image.The split rule may be predetermined between the image decoding apparatus100 and the image encoding apparatus 200. The image decoding apparatus100 may determine the split rule of the image, based on informationobtained from a bitstream. The image decoding apparatus 100 maydetermine the split rule based on the information obtained from at leastone of a sequence parameter set, a picture parameter set, a videoparameter set, a slice header, a slice segment header, a tile header,and a tile group header. The image decoding apparatus 100 may determinethe split rule differently according to frames, slices, tiles, temporallayers, largest coding units, or coding units.

The image decoding apparatus 100 may determine the split rule based on ablock shape of a coding unit. The block shape may include a size, shape,a ratio of width and height, and a direction of the coding unit. Theimage encoding apparatus 200 and the image decoding apparatus 100 maypredetermine to determine the split rule based on the block shape of thecoding unit. However, the embodiment is not limited thereto. The imagedecoding apparatus 100 may determine the split rule based on theinformation obtained from the bitstream received from the image encodingapparatus 200.

The shape of the coding unit may include a square and a non-square. Whenthe lengths of the width and height of the coding unit are the same, theimage decoding apparatus 100 may determine the shape of the coding unitto be a square. Also, when the lengths of the width and height of thecoding unit are not the same, the image decoding apparatus 100 maydetermine the shape of the coding unit to be a non-square.

The size of the coding unit may include various sizes, such as 4×4, 8×4,4×8, 8×8, 16×4, 16×8, and to 256×256. The size of the coding unit may beclassified based on the length of a long side of the coding unit, thelength of a short side, or the area. The image decoding apparatus 100may apply the same split rule to coding units classified as the samegroup. For example, the image decoding apparatus 100 may classify codingunits having the same lengths of the long sides as having the same size.Also, the image decoding apparatus 100 may apply the same split rule tocoding units having the same lengths of long sides.

The ratio of the width and height of the coding unit may include 1:2,2:1, 1:4, 4:1, 1:8, 8:1, 1:16, 16:1, 32:1, 1:32, or the like. Also, adirection of the coding unit may include a horizontal direction and avertical direction. The horizontal direction may indicate a case inwhich the length of the width of the coding unit is longer than thelength of the height thereof. The vertical direction may indicate a casein which the length of the width of the coding unit is shorter than thelength of the height thereof.

The image decoding apparatus 100 may adaptively determine the split rulebased on the size of the coding unit. The image decoding apparatus 100may differently determine an allowable split shape mode based on thesize of the coding unit. For example, the image decoding apparatus 100may determine whether splitting is allowed based on the size of thecoding unit. The image decoding apparatus 100 may determine a splitdirection according to the size of the coding unit. The image decodingapparatus 100 may determine an allowable split type according to thesize of the coding unit.

The split rule determined based on the size of the coding unit may be asplit rule predetermined between the image encoding apparatus 200 andthe image decoding apparatus 100. Also, the image decoding apparatus 100may determine the split rule based on the information obtained from thebitstream.

The image decoding apparatus 100 may adaptively determine the split rulebased on a location of the coding unit. The image decoding apparatus 100may adaptively determine the split rule based on the location of thecoding unit in the image.

Also, the image decoding apparatus 100 may determine the split rule suchthat coding units generated via different splitting paths do not havethe same block shape. However, an embodiment is not limited thereto, andthe coding units generated via different splitting paths have the sameblock shape. The coding units generated via the different splittingpaths may have different decoding processing orders. Because thedecoding processing orders is described above with reference to FIG. 12, details thereof are not provided again.

FIG. 17 illustrates coding units of individual pictures, when theindividual pictures have different split shape combinations of codingunits, according to an embodiment.

Referring to FIG. 17 , the image decoding apparatus 100 may determinedifferent split shape combinations of coding units for individualpictures. For example, the image decoding apparatus 100 may decode animage by using a picture 1700 that can be split into four coding units,a picture 1710 that can be split into two or four coding units, and apicture 1720 that can be split into two, three, or four coding units,among at least one picture included in the image. The image decodingapparatus 100 may use only split shape information indicating splittinginto four square coding units, in order to split the picture 1700 into aplurality of coding units. The image decoding apparatus 100 may use onlysplit shape information indicating splitting into two or four codingunits, in order to split the picture 1710. The image decoding apparatus100 may use only split shape information indicating splitting into two,three, or four coding units, in order to split the picture 1720. Theabove-described split shape combinations are embodiments for describingoperations of the image decoding apparatus 100, and therefore, theabove-described split shape combinations should not be interpreted to belimited to the above-described embodiments. It should be interpretedthat various split shape combinations can be used for each preset dataunit.

According to an embodiment, the bitstream obtainer 110 of the imagedecoding apparatus 100 may obtain a bitstream including an indexrepresenting a combination of split shape information for each presetdata unit (for example, a sequence, a picture, a slice, a slice segment,a tile, a tile group, etc.). For example, the bitstream obtainer 110 mayobtain an index representing a combination of split shape informationfrom a sequence parameter set, a picture parameter set, a slice header,a tile header, or a tile group header. The bitstream obtainer 110 of theimage decoding apparatus 100 may use the obtained index to determine asplit shape combination into which coding units can be split for eachpreset data unit, and accordingly, the bitstream obtainer 110 may usedifferent split shape combinations for individual preset data units.

FIG. 18 illustrates various shapes of coding units that can bedetermined based on split shape mode information that can be expressedwith a binary code, according to an embodiment.

According to an embodiment, the image decoding apparatus 100 may splitcoding units into various shapes by using block shape information andsplit shape mode information obtained through the bitstream obtainer110. Shapes into which coding units can be split may be various shapesincluding shapes described above through the embodiments.

Referring to FIG. 18 , the image decoding apparatus 100 may split acoding unit having a square shape in at least one direction of ahorizontal direction and a vertical direction, and a coding unit havinga non-square shape in the horizontal direction or the verticaldirection, based on split shape mode information.

According to an embodiment, when the image decoding apparatus 100 cansplit a coding unit having a square shape in the horizontal directionand the vertical direction to determine four square coding units, splitshape mode information for a square coding unit may represent four splitshapes. According to an embodiment, the split shape mode information maybe expressed with a binary code of 2 digits, and each split shape may beassigned a binary code. For example, when a coding unit is not split,split shape mode information may be expressed as (00)b, when a codingunit is split in the horizontal direction and the vertical direction,split shape mode information may be expressed as (01)b, when a codingunit is split in the horizontal direction, split shape mode informationmay be expressed as (10)b, and when a coding unit is split in thevertical direction, split shape mode information may be expressed as(11)b.

According to an embodiment, when the image decoding apparatus 100 splitsa coding unit having a non-square shape in the horizontal direction orthe vertical direction, kinds of split shapes that can be represented bysplit shape mode information may depend on the number of coding unitsinto which the coding unit is to be split. Referring to FIG. 18 , theimage decoding apparatus 100 may split a coding unit having a non-squareshape up to three, according to an embodiment. Also, the image decodingapparatus 100 may split a coding unit into two coding units. In thiscase, split shape mode information may be expressed as (10)b. The imagedecoding apparatus 100 may split a coding unit into three coding units.In this case, split shape mode information may be expressed as (11)b.The image decoding apparatus 100 may determine not to split a codingunit. In this case, split shape mode information may be expressed as(0)b. That is, the image decoding apparatus 100 may use Variable LengthCoding (VLC), instead of Fixed Length Coding (FLC), in order to use abinary code representing split shape mode information.

According to an embodiment, referring to FIG. 18 , a binary code ofsplit shape mode information representing that a coding unit is notsplit may be expressed as (0)b. In the case in which a binary code ofsplit shape mode information representing that a coding unit is notsplit is set to (00)b, a binary code of split shape mode information of2 bits may need to be all used although there is no split shape modeinformation set to (01)b. However, in the case in which three splitshapes are used for a coding unit having a non-square shape, as shown inFIG. 18 , the image decoding apparatus 100 can determine that a codingunit is not split by using a binary code (0)b of 1 bit as split shapemode information, thereby efficiently using a bitstream. However, splitshapes of a coding unit having a non-square shape, which are representedby split shape mode information, should be not interpreted to be limitedto three shapes shown in FIG. 18 , and should be interpreted to bevarious shapes including the above-described embodiments.

FIG. 19 illustrates other shapes of coding units that can be determinedbased on split shape mode information that can be represented with abinary code, according to an embodiment.

Referring to FIG. 19 , the image decoding apparatus 100 may split acoding unit having a square shape in the horizontal direction or thevertical direction, and a coding unit having a non-square shape in thehorizontal direction or the vertical direction, based on split shapemode information. That is, the split shape mode information may indicatesplitting a coding unit having a square shape in one direction. In thiscase, a binary code of split shape mode information representing that acoding unit having a square shape is not split may be expressed as (0)b.In the case in which a binary code of split shape mode informationrepresenting that a coding unit is not split is set to (00)b, a binarycode of split shape mode information of 2 bits may need to be all usedalthough there is no split shape mode information set to (01)b. However,in the case in which three split shapes are used for a coding unithaving a square shape, as shown in FIG. 19 , the image decodingapparatus 100 can determine that a coding unit is not split by using abinary code (0)b of 1 bit as split shape mode information, therebyefficiently using a bitstream. However, split shapes of a coding unithaving a square shape, which are represented by split shape modeinformation, should be not interpreted to be limited to three shapesshown in FIG. 19 , and should be interpreted to be various shapesincluding the above-described embodiments.

According to an embodiment, block shape information or split shape modeinformation may be expressed by using a binary code, and the block shapeinformation or split shape mode information may be generated directly asa bitstream. Also, block shape information or split shape modeinformation that can be expressed with a binary code may be used as aninput binary code in context adaptive binary arithmetic coding (CAB AC),instead of being generated directly as a bitstream.

A process in which the image decoding apparatus 100 obtains a syntax forblock shape information or split shape mode information through CABAC,according to an embodiment, will be described. The image decodingapparatus 100 may obtain a bitstream including a binary code for thesyntax through the bitstream obtainer 110. The image decoding apparatus100 may de-binarize a bin string included in the obtained bitstream todetect a syntax element representing block shape information or splitshape mode information. According to an embodiment, the image decodingapparatus 100 may obtain a group of binary bin strings corresponding toa syntax element to be decoded, and decode the individual bins by usingprobability information. The image decoding apparatus 100 may repeat theoperation until a bin string configured with the decoded bins isidentical to one of previously obtained bin strings. The image decodingapparatus 100 may perform de-binarization on the bin string to determinea syntax element.

According to an embodiment, the image decoding apparatus 100 may performa decoding process of adaptive binary arithmetic coding to determine asyntax for the bin string, and the image decoding apparatus 100 mayupdate a probability model for the bins obtained through the bitstreamobtainer 110. Referring to FIG. 18 , the bitstream obtainer 110 of theimage decoding apparatus 100 may obtain a bitstream that represents abinary code representing split shape mode information, according to anembodiment. The image decoding apparatus 100 may determine a syntax forthe split shape mode information by using the obtained binary codehaving a size of 1 or 2 bits. The image decoding apparatus 100 mayupdate a probability for each bit of the binary code of 2 bits, in orderto determine the syntax for the split shape mode information. That is,the image decoding apparatus 100 may update, according to which one of 0or 1 a value of a first bin of the binary code of 2 bits is, aprobability that the next bin will have a value of 0 or 1 upon decoding.

According to an embodiment, in the process of determining the syntax,the image decoding apparatus 100 may update probabilities for the binsthat are used in a process of decoding the bins of the bin string forthe syntax, and the image decoding apparatus 100 may determine that aspecific bit of the bin string has the same probability, withoutupdating a probability of the specific bit.

Referring to FIG. 18 , in a process of determining a syntax by using abin string representing split shape mode information for a coding unithaving a non-square shape, the image decoding apparatus 100 maydetermine a syntax for the split shape mode information by using a binhaving a value of 0 in the case in which the coding unit having thenon-square shape is not split. That is, when block shape informationrepresents that a current coding unit has a non-square shape, a firstbin of the bin string for the split shape mode information may be 0 inthe case in which the coding unit having the non-square shape is notsplit, and may be 1 in the case in which the coding unit is split intotwo or three coding units. Accordingly, a probability that the first binof the bin string of the split shape mode information for the codingunit having the non-square shape will be 0 may be ⅓, and a probabilitythat the first bin will be 1 may be ⅔. Because split shape modeinformation representing that a coding unit having a non-square shape isnot split can be expressed with a bin string of 1 bit having a value of0, as described above, the image decoding apparatus 100 may determine,only in the case in which the first bin of the split shape modeinformation is 1, whether a second bin is 0 or 1 to determine the syntaxfor the split shape mode information. According to an embodiment, whenthe first bin for the split shape mode information is 1, the imagedecoding apparatus 100 may determine that a probability that the secondbin will be 0 is equal to a probability that the second bin will be 1,and decode the second bin.

Accordingly, the image decoding apparatus 100 may use, in the process ofdetermining the bins of the bin string for the split shape modeinformation, various probabilities for the individual bins. According toan embodiment, the image decoding apparatus 100 may determine differentprobabilities of bins for split shape mode information according to anextension direction of a non-square block. According to an embodiment,the image decoding apparatus 100 may determine different probabilitiesof bins for split shape mode information according to a width of acurrent coding unit or a length of a longer side of the current codingunit. According to an embodiment, the image decoding apparatus 100 maydetermine different probabilities of bins for split shape modeinformation according to at least one of a shape of a current codingunit and a length of a longer side of the current coding unit.

According to an embodiment, the image decoding apparatus 100 maydetermine that probabilities of bins for split shape mode informationare the same with respect to coding units that are equal to or largerthan a preset size. For example, the image decoding apparatus 100 maydetermine that probabilities of bins for split shape mode informationare the same with respect to coding units of which lengths of longersides are equal to or greater than 64 samples.

According to an embodiment, the image decoding apparatus 100 maydetermine initial probabilities for bins constituting a bin string ofsplit shape mode information based on a slice type (for example, an Islice, a P slice, or a B slice).

FIG. 20 is a block diagram of an image encoding and decoding system thatperforms loop filtering.

An encoder 2010 of an image encoding and decoding system 2000 maytransmit an encoded bitstream of an image, and a decoder 2050 of theimage encoding and decoding system 2000 may receive a bitstream anddecode the bitstream to output a reconstructed image. Herein, theencoder 2010 may be a configuration that is similar to the imageencoding apparatus 200 which will be described at a later time, and thedecoder 2050 may be a configuration that is similar to the imagedecoding apparatus 100.

In the encoder 2010, a prediction encoder 2015 may output predictiondata through inter prediction and intra prediction, and a transformationand quantization unit 2020 may output a quantized transform coefficientof residual data between the prediction data and a current input image.An entropy encoder 2025 may encode the quantized transform coefficientto transform the quantized transform coefficient, and output thequantized transform coefficient as a bitstream. The quantized transformcoefficient may be reconstructed as spatial-domain data through adequantization and inverse-transformation unit 2030, and thereconstructed spatial-domain data may be output as a reconstructed imagethrough a deblocking filter 2035 and a loop filter 2040. Thereconstructed image may be used as a reference image of a next inputimage by a prediction encoder 2015.

Encoded image data of a bitstream received by the decoder 2050 may bereconstructed as spatial-domain residual data through an entropy decoder2055 and a dequantization and inverse-transformation unit 2060.Prediction data output from a prediction decoder 2075 may be combinedwith the residual data to construct spatial-domain image data, and adeblocking filter 2065 and a loop filter 2070 may filter thespatial-domain image data and output a reconstructed image for a currentoriginal image. The reconstructed image may be used as a reference imagefor a next original image by the prediction decoder 2075.

The loop filter 2040 of the encoder 2010 may perform loop filtering byusing filter information input according to a user input or a systemsetting. The filter information used by the loop filter 2040 may beoutput to the entropy encoder 2025, and transmitted to the decoder 2050together with encoded image data. The loop filter 2070 of the decoder2050 may perform loop filtering based on filter information input fromthe decoder 2050.

Various embodiments described above describe operations related to animage decoding method that is performed by the image decoder 100.Hereinafter, operations of the image encoding apparatus 200 thatperforms an image encoding method corresponding to a reverse order ofthe image decoding method will be described through various embodiments.

FIG. 2 is a block diagram of the image encoding apparatus 200 capable ofencoding an image based on at least one of block shape information andsplit shape mode information, according to an embodiment.

The image encoding apparatus 200 may include an encoder 220 and abitstream generator 210. The encoder 220 may receive an input image andencode the input image. The encoder 220 may encode the input image toobtain at least one syntax element. The syntax element may include atleast one of a skip flag, a prediction mode, a motion vector difference,a motion vector prediction method (or index), a transform quantizedcoefficient, a coded block pattern, a coded block flag, an intraprediction mode, a direct flag, a merge flag, a delta QP, a referenceindex, a prediction direction, and a transform index. The encoder 220may determine a context model based on block shape information includingat least one of a shape, a direction, a ratio of a height and a width,or a size of a coding unit.

The bitstream generator 210 may generate a bitstream based on an encodedinput image. For example, the bitstream generator 210 may generate abitstream by performing entropy encoding on a syntax element based on acontext model. Also, the image encoding apparatus 200 may transmit thebitstream to the image decoding apparatus 100.

According to an embodiment, the encoder 220 of the image encodingapparatus 200 may determine a shape of a coding unit. For example, acoding unit may have a square shape or a non-square shape, andinformation representing such a shape may be included in block shapeinformation.

According to an embodiment, the encoder 220 may determine a shape intowhich a coding unit is to be split. The encoder 220 may determine ashape of at least one coding unit included in a coding unit, and thebitstream generator 210 may generate a bitstream including split shapemode information including information about the shape of the codingunit.

According to an embodiment, the encoder 220 may determine whether or notto split a coding unit. When the encoder 220 determines that a codingunit includes only one coding unit or that a coding unit is not split,the bitstream generator 210 may generate a bitstream including splitshape mode information representing that the coding unit is not split.Also, the encoder 220 may split a coding unit into a plurality of codingunits included in the coding unit, and the bitstream generator 210 maygenerate a bitstream including split shape mode information representingthat a coding unit is to be split into a plurality of coding units.

According to an embodiment, information representing the number ofcoding units into which a coding unit is split or a direction in whichthe coding unit is split may be included in the split shape modeinformation. For example, the split shape mode information may representsplitting in at least one direction of a vertical direction and ahorizontal direction or may represent non-splitting.

The image encoding apparatus 200 may determine split shape modeinformation based on a split shape mode of a coding unit. The imageencoding apparatus 200 may determine a context model based on at leastone of a shape, a direction, a ratio of a width and a height, or a sizeof the coding unit. Also, the image encoding apparatus 200 may generateinformation about a split shape mode for splitting the coding unit as abitstream based on the context model.

To determine the context model, the image encoding apparatus 200 mayobtain an arrangement for corresponding at least one of a shape, adirection, a ratio of a width and a height, or a size of the coding unitto an index for the context model. The image encoding apparatus 200 mayobtain the index for the context model based on at least one of theshape, the direction, the ratio of the width and the height, or the sizeof the coding unit, from the arrangement. The image encoding apparatus200 may determine the context model based on the index for the contextmodel.

To determine the context model, the image encoding apparatus 200 maydetermine the context model further based on block shape informationincluding at least one of a shape, a direction, a ratio of a width and aheight, or a size of a surrounding coding unit being adjacent to thecoding unit. Also, the surrounding coding unit may include at least oneof coding units located to the left-lower side, left side, left-upperside, upper side, right-upper side, right side, or right-lower side ofthe coding unit.

Also, to determine the context model, the image encoding apparatus 200may compare a length of a width of the upper surrounding coding unitwith a length of the width of the coding unit. Also, the image encodingapparatus 200 may compare lengths of heights of the left and rightsurrounding coding units with a length of the height of the coding unit.Also, the image encoding apparatus 200 may determine the context modelbased on results of the comparisons.

Operations of the image encoding apparatus 200 include content that issimilar to those of the image decoding apparatus 100 described abovewith reference to FIGS. 3 to 20 , and therefore, detailed descriptionsthereof will be omitted.

Hereinafter, an image encoding apparatus 3000 and an image decodingapparatus 2100 for encoding and decoding an image through bi-directionalprediction will be described with reference to FIGS. 21 to 32 .

FIG. 21 is a block diagram illustrating a configuration of the imagedecoding apparatus 2100 according to an embodiment.

Referring to FIG. 21 , the image decoding apparatus 2100 according to anembodiment may include an obtainer 2110, an entropy decoder 2130, and aprediction decoder 2150.

The obtainer 2110 shown in FIG. 21 may correspond to the bitstreamobtainer 110 shown in FIG. 1 , and the entropy decoder 2130 and theprediction decoder 2150 may correspond to the decoder 120 shown in FIG.1 . Also, the entropy decoder 2130 and the prediction decoder 2150 mayrespectively correspond to the entropy decoder 2055 and the predictiondecoder 2075 shown in FIG. 20 .

The obtainer 2110, the entropy decoder 2130, and the prediction decoder2150, according to an embodiment, may be implemented as at least oneprocessor. The image decoding apparatus 2100 may include one or morememories (not shown) for storing input and output data of the obtainer2110, the entropy decoder 2130, and the prediction decoder 2150. Also,the image decoding apparatus 2100 may include a memory controller (notshown) for controlling data inputs and outputs of the memory (notshown).

The obtainer 2110 may receive a bitstream generated as a result ofencoding of an image. The bitstream may include information to be usedto reconstruct a current block. The current block may be a blockgenerated by being split from the image according to a tree structure,and may correspond to a block unit, such as, for example, a largestcoding unit, a coding unit, or a transform unit, etc.

The prediction decoder 2150 may determine a current block based on blockshape information and/or split shape mode information included in abitstream corresponding to at least one level among a sequence parameterset, a picture parameter set, a video parameter set, a slice header, anda slice segment header.

The bitstream may include information representing a prediction mode ofthe current block. The prediction mode of the current block may includean intra mode, an inter mode, a merge mode, etc. The inter mode, themerge mode, etc. may be modes for predicting and reconstructing acurrent block based on a reference image in order to reduce temporalredundancy between images.

According to an embodiment, a reference image may be used (that is,uni-directional prediction) or two reference images may be used (thatis, bi-directional prediction) to reconstruct a current block based on areference image(s). Whether a current block will be subject touni-directional prediction or bi-directional prediction may bedetermined according to explicit information included in a bitstream, orimplicitly determined from a prediction mode of surrounding blocksrelated to the current block.

The entropy decoder 2130 may perform entropy decoding on data includedin the bitstream to obtain syntax elements to be used for reconstructingthe current block. According to an embodiment, the entropy decoder 2130may perform entropy decoding on the data included in the bitstreamaccording to CABAC.

When the current block is subject to bi-directional prediction, theentropy decoder 2130 may perform entropy decoding on weight informationincluded in the bitstream to obtain a weight index corresponding to asyntax element. Herein, the weight information may include a resultvalue obtained by performing arithmetic encoding on a binary stringobtained by binarizing the weight index. Accordingly, as a result ofarithmetic decoding of the weight information, the binary stringcorresponding to the weight index may be reconstructed, and the binarystring may be de-binarized to obtain the weight index. The binary stringobtained by binarizing the weight index and the binary string obtainedby performing arithmetic decoding on the weight information may bereferred to as bin strings.

According to an embodiment, the entropy decoder 2130 may reconstruct,upon arithmetic decoding of weight information, a first binary value ofa binary string corresponding to a weight index based on a contextmodel, and reconstruct the remaining binary values of the binary stringby a bypass method. That is, because the context model is used only forthe first binary value, entropy decoding may be simplified.

The entropy decoder 2130 will be described in detail with reference toFIGS. 22 to 24 .

FIG. 22 illustrates a configuration of the entropy decoder 2130 shown inFIG. 21 .

Referring to FIG. 22 , the entropy decoder 2130 may include a contextmodeler 2132, a regular decoder 2134, a bypass decoder 2136, and ade-binarizer 2138. The entropy decoder 2130 may perform a reverseprocess of an entropy encoding process that is performed by an entropyencoder 3030 which will be described later.

A bitstream may be configured with binary values, and the binary valuesmay be obtained by performing arithmetic encoding on a binary stringcorresponding to syntax elements determined during an image encodingprocess.

Data included in the bitstream may be arithmetically decoded to binaryvalues (that is, the binary string) corresponding to the syntax elementsthrough the regular decoder 2134 or the bypass decoder 2136. The dataincluded in the bitstream may be input to the regular decoder 2134 orthe bypass decoder 2136 according to types of the syntax elements.

The regular decoder 2134 may perform arithmetic decoding on the binaryvalues based on a probability model determined by the context modeler2132. The context modeler 2132 may provide the probability model for thebinary values to be currently reconstructed, to the regular decoder2134. More specifically, the context modeler 2132 may determine aprobability of a preset binary value based on a previously decodedbinary value, update a probability of a binary value used to decode thepreviously decoded binary value, and output the updated probability tothe regular decoder 2134.

According to an embodiment, the context modeler 2132 may determine acontext model by using a context index ctxldx, and determine anoccurrence probability of a Least Probable Symbol (LPS) or a MostProbable Symbol (MPS) which the context model has, and informationvalMPS about which one of 0 and 1 corresponds to an MPS. According toanother embodiment, the context modeler 2132 may determine a probability(for example, P(1) representing an occurrence probability of, forexample, “1”) of a predetermined, preset binary value based onpreviously decoded binary values, without distinguishing an MPS from anLPS, and provide the probability of the preset binary value to theregular decoder 2134.

The regular decoder 2134 may perform binary arithmetic decoding based onthe probability of the preset binary value provided from the contextmodeler 2132 and the data included in the bitstream. More specifically,the regular decoder 2134 may determine an occurrence probability P(1) of“1” and an occurrence probability P(0) of “0” based on the probabilityof the preset binary value provided from the context modeler 2132. Also,the regular decoder 2134 may split a preset range representing aprobability section according to the occurrence probabilities P(0) andP(1) of “0” and “1”, and output a binary value corresponding to asection to which the data included in the bitstream belongs.

The bypass decoder 2136 may fix the occurrence probability P(1) of “1”and the occurrence probability P(0) of “0” to a predetermined value, forexample, 0.5, split a preset range representing a probability sectionaccording to the occurrence probabilities P(1) and P(0), and output abinary value corresponding to a section to which the data included inthe bitstream belongs. Because the bypass decoder 2136 does not use acontext model unlike the regular decoder 2134, the bypass decoder 2136may perform high-speed arithmetic decoding.

The de-binarizer 2138 may de-binarize a binary string being anarrangement of binary values output from the regular decoder 2134 andthe bypass decoder 2136 to output a syntax element. For thede-binarization of the binary string, one of fixed lengthde-binarization, truncated rice de-binarization, k-th order exp-golombde-binarization, and golomb rice de-binarization may be used.

As described above, the entropy decoder 2130 may reconstruct, uponarithmetic decoding of weight information included in a bitstream, afirst binary value based on a context model and reconstruct theremaining binary values by a bypass method. In other words, a firstbinary value of a binary string corresponding to a weight index may bereconstructed by the regular decoder 2134, and the remaining binaryvalue may be reconstructed by the bypass decoder 2136.

FIG. 23 is a reference table for determining context information of abinary string corresponding to a weight index.

Referring to FIG. 23 , it is seen that a binary value (that is, a firstbinary value) having binldx of 0 among binary values of Bcw_idxrepresenting weight indexes is reconstructed according to context of 0,and binary values having binldx that is greater than 0 are reconstructedby the bypass method.

After the arithmetic decoding of the weight information is completed bythe entropy decoder 2130, a weight index corresponding to a syntaxelement may be obtained through de-binarization of the binary string.

FIG. 24 is a table representing candidate values included in a weightcandidate group and weight indexes and binary strings corresponding tothe candidate values.

Referring to FIG. 24 , weight indexes may be assigned to candidatevalues used for bi-directional prediction of a current block, and theindividual weight indexes may be binarized by the truncated ricebinarization. For example, a weight index 0 may be expressed with abinary value, and a weight index 1 may be expressed with two binaryvalues. That is, weight indexes may be expressed with different numbersof binary values. However, according to an implementation example, aweight index having a greatest value and a weight index having asecond-greatest value may be expressed with the same number of binaryvalues based on the truncated rice binarization. The kinds/numbers ofcandidate values shown in FIG. 24 and weight indexes assigned to therespective candidate values are examples, and may change variouslywithin a range that is apparent to one of ordinary skill in the relatedart.

It has been described above that, upon arithmetic decoding of weightinformation, a first binary value is reconstructed based on a contextmodel and the remaining binary values are reconstructed by the bypassmethod. The reason may be because i) when a selection probability of acandidate value indicated by a weight index 0 is highest, it isefficient to reconstruct a first binary value by accumulating changes ofa probability of 0 and a probability of 1 according to a context model,and ii) because selection probabilities of candidate values indicated byother weight indexes except for 0 are lower than the selectionprobability of the candidate value indicated by the weight index 0, andaccordingly, it is more efficient to reconstruct binary values by thebypass method than to reconstruct the binary values based on a contextmodel.

The prediction decoder 2150 may use a candidate value indicated by aweight index obtained as a result of entropy decoding of weightinformation for bi-directional prediction of a current block. Morespecifically, when the weight index is 0, a candidate value of 4 may beused for bi-directional prediction of a current block, and, when theweight index is 1, a candidate value of 5 may be used for bi-directionalprediction of a current block.

Hereinafter, a method performed by the prediction decoder 2150 ofreconstructing a current block through bi-directional prediction using aweight will be described.

FIG. 25 is a view for describing bi-directional prediction of a currentblock 2515.

The current block 2515 included in a current image 2510 may be subjectto uni-directional prediction by using a first reference image 2530included in a list 0 or a second reference image 2550 included in a list1, or may be subject to bi-directional prediction by using the firstreference image 2530 included in the list 0 and the second referenceimage 2550 included in the list 1.

The prediction decoder 2150 may determine the first reference image 2530and the second reference image 2550 to be referred to by the currentblock 2515 for bi-directional prediction of the current block 2515, anddetermine a first motion vector mv1 indicating the first reference block2535 in the first reference image 2530 and a second motion vector mv2indicating the second reference block 2555 in the second reference image2550. More specifically, the prediction decoder 2150 may select thefirst reference image 2530 and the second reference image 2550 as imagesto be referred to by the current block 2515 based on informationincluded in a bitstream, or the prediction decoder 2150 may select thefirst reference image 2530 and the second reference image 2550 as imagesto be referred to by the current block 2515 in consideration of imagesreferred to by surrounding blocks related to the current block 2515.

To determine the first motion vector mv1 and the second motion vectormv2, the prediction decoder 2150 may generate a motion vector candidatelist by using motion vectors of a temporal block temporally related tothe current block 2515 and a spatial block spatially related to thecurrent block 2515. The prediction decoder 2150 may use the first motionvector mv1 and the second motion vector mv2 by using a motion vectorcandidate indicated by the information included in the bitstream amongmotion vector candidates included in the motion vector candidate list.

FIG. 26 illustrates blocks temporally and/or spatially related to thecurrent block 2515.

Referring to FIG. 26 , a temporal block may include at least one of ablock Col having a different Picture Order Count (POC) from a POC of thecurrent block 2515 and positioned at the same location as the currentblock 2515 in a reference image, and a block Br being spatially adjacentto the block Col positioned at the same location. The block BR may bepositioned to the right lower side of the block Col positioned at thesame location as the current block 2515.

A spatial block spatially related to the current block 2515 may includeat least one of a left lower outside block AO, a left lower block A1, aright upper outside block B0, an upper right block B1, and a left upperoutside block B2.

Locations of temporal blocks and spatial blocks shown in FIG. 26 areexamples, and the locations and numbers of the temporal blocks andspatial blocks may change variously according to an implementationexample.

According to an embodiment, the prediction decoder 2150 may search forthe first reference block 2535 and the second reference block 2555 to beused to reconstruct the current block 2515 directly in the firstreference image 2530 and the second reference image 2550. In this case,the prediction decoder 2150 may search for the first reference block2535 and the second reference block 2555 by the same method as thatperformed by the image encoding apparatus 3000.

Referring again to FIG. 25 , after the first reference block 2535 in thefirst reference image 2530 and the second reference block 2555 in thesecond reference image 2550 are determined, the prediction decoder 2150may combine the first reference block 2535 with the second referenceblock 2555, and reconstruct the current block 2515 based on the combinedresult. Herein, combining the first reference block 2535 with the secondreference block 2555 may mean linearly combining samples included in thefirst reference block 2535 with samples included in the second referenceblock 2555.

For example, the prediction decoder 2150 may determine the combinedresult of the first reference block 2535 and the second reference block2555, as the current block 2515. As another example, the predictiondecoder 2150 may apply residual data obtained from a bitstream to thecombined result of the first reference block 2535 and the secondreference block 2555, thereby reconstructing the current block 2515.Herein, the residual data may represent a difference between thecombined result of the first reference block 2535 and the secondreference block 2555 and the current block 2515.

The prediction decoder 2150 may combine the first reference block 2535with the second reference block 2555 according to Equation 1 below.

pbSamples[x][y]=(w0*predSamplesL0[x][y]+w1*predSamplesL1[x][y]+offset3)>>>(shift2+3)  [Equation1]

In Equation 1, pbSamples[x][y] represents a combined result of a samplepositioned at a location (x, y) in the first reference block 2535 and asample positioned at a location (x, y) in the second reference block2555, predSamplesL0[x][y] represents a sample positioned at a location(x, y) of a first reference block in the first reference image 2530included in the list 0, and predSamplesL1[x] [y] represents a samplepositioned at a location (x, y) of a second reference block in thesecond reference image 2550 included in the list 1. Also, offset3 andshift2 are predetermined values.

In Equation 1, w1 represents a candidate value corresponding to a weightindex, and w0 represents a pair value of w1. A pair value means a valueobtained by applying a candidate value corresponding to a weight indexto a preset operation equation. For example, w0 may have a value of8−w1. According to an implementation example, w0 may be a candidatevalue corresponding to a weight index, and w1 may be a pair value of w0.

As described above, when a candidate value indicated by a weight indexamong candidate values included in a weight candidate group isconfirmed, the corresponding candidate value may be used as a weight forcombining a first reference block with a second reference block. Thatis, according to an embodiment, by selecting, when a first referenceblock is combined with a second reference block for bi-directionalprediction of a current block, a weight by which a combined result beingmost similar to the current block can be deduced, a size of residualdata included in a bitstream may be reduced.

Meanwhile, because indexes respectively indicating candidate valuesincluded in a weight candidate group are binarized by, for example, thetruncated rice binarization to be expressed with different numbers ofbinary values, which indexes are assigned to which candidate values maybe important in view of a bitrate. In other words, when a smallest indexis assigned to a candidate value having a greatest probability to beused for bi-directional prediction of a current block among candidatevalues included in a weight candidate group, the corresponding index maybe expressed with a small number of bits. For example, when a candidatevalue having a greatest probability to be used for bi-directionalprediction of a current block among the candidate values shown in FIG.24 is −2 and an index of 4 is assigned to the candidate value of −2,four binary values (that is, 1111) may be needed to express the index of4, and therefore, a disadvantage exists in view of a bitrate comparedwith a case of assigning an index of 0 to the candidate value of −2.

Accordingly, the prediction decoder 2150 according to an embodiment mayadaptively assign indexes to candidate values included in a weightcandidate group. Hereinafter, adaptively assigning indexes means thatindexes to be assigned to candidate values can change according topreset criterion, instead of respectively applying the same indexes tothe candidate values. Hereinafter, this will be described in detail.

According to an embodiment, the prediction decoder 2150 may adaptivelyset indexes for candidate values according to an accumulative number oftimes which the candidate values have been selected, for bi-directionalprediction of previous blocks decoded earlier than a current block. Forexample, the prediction decoder 2150 may assign indexes of smallervalues to candidate values having greater accumulative numbers of timeswhich the candidate values have been selected.

FIG. 27 is a view for describing that indexes of candidate values canchange according to accumulative numbers of times which the candidatevalues included in a weight candidate group have been selected.

As shown in FIG. 27 , when a candidate value 4 has been selected 19times, a candidate value 5 has been selected 6 times, a candidate value3 has been selected 12 times, a candidate value 10 has been selected 3times, and a candidate value −2 has been selected 7 times in previousblocks, indexes of values increasing in an order of the candidate values4, 3, −2, 5, and 10 may be respectively assigned to the correspondingcandidate values.

The prediction decoder 2150 may assign indexes to candidate values,respectively, and combine a first reference block with a secondreference block by using a candidate value corresponding to a weightindex, as described above.

According to an embodiment, the prediction decoder 2150 may newly assignindexes to candidate values, respectively, for each image, each slice,each tile, or each block. Herein, a block may include a largest codingunit, a coding unit, or a transform unit.

For example, when indexes are newly assigned to candidate values in unitof an image, the prediction decoder 2150 may calculate, before decodinga current image including a current block, accumulative numbers of timeswhich the candidate values have been selected for bi-directionalprediction of previous blocks included in previous images, and assignindexes to candidate values to be used for bi-directional prediction ofblocks in a current image according to the accumulative numbers oftimes.

According to another example, when indexes are newly assigned tocandidate values in unit of a slice, the prediction decoder 2150 maycalculate, before decoding a current slice including a current block,accumulative numbers of times which the candidate values have beenselected for bi-directional prediction of previous blocks included inprevious slices, and assign indexes to candidate values to be used forbi-directional prediction of blocks in a current slice according to thecalculated accumulative numbers of times.

According to another example, when indexes are newly assigned tocandidate values in unit of a tile, the prediction decoder 2150 maycalculate, before decoding a current tile including a current block,accumulative numbers of times which the candidate values have beenselected for bi-directional prediction of previous blocks included inprevious tiles, and assign indexes to candidate values to be used forbi-directional prediction of blocks in the current tile according to thecalculated accumulative numbers of times.

According to another example, when indexes are newly assigned tocandidate values in unit of a block, the prediction decoder 2150 maycalculate, before decoding a current block, accumulative numbers oftimes which the candidate values have been selected for bi-directionalprediction of previous blocks, and assign indexes to candidate values tobe used for bi-directional prediction of the current block according tothe calculated accumulative numbers of times.

According to an embodiment, the prediction decoder 2150 may selectbi-directionally predicted previous blocks based on a first referenceimage and a second reference image used for bi-directional prediction ofa current block among previous blocks, and set indexes for candidatevalues according to accumulative numbers of times which the candidatevalues have been selected, for bi-directional prediction of thecorresponding previous blocks. In other words, accumulative numbers oftimes which candidate values have been selected may be calculated withrespect to previous blocks bi-directionally predicted by using the samereference image as a reference image of a current image, and indexes maybe assigned according to the accumulative numbers of times. In thiscase, the prediction decoder 2150 may specify a reference image referredto by a current block from information (for example, ref_idx) indicatingreference images used for bi-directional prediction of the currentblock, and then select a candidate value indicated by a weight index(for example, bcw_idx). Accordingly, the entropy decoder 2130 mayreconstruct the information (for example, ref_idx) indicating thereference images used for bi-directional prediction of the current blockfrom a bitstream, and then reconstruct the weight index.

When there is no previous block bi-directionally predicted based on thefirst reference image and the second reference image used forbi-directional prediction of the current block, the prediction decoder2150 may assign indexes of predetermined values to candidate values. Thereason may be because there is a high probability that the same weightwill be selected to combine reference blocks, with respect to blocksbi-directionally predicted based on the same reference image.

According to an embodiment, the prediction decoder 2150 may select aprevious image having the same temporal layer as that of a current imagein a current Group of Picture (GOP) including the current image fromamong previous images, and set, for bi-directional prediction ofprevious blocks included in the selected previous image, indexes tocandidate values according to accumulative numbers of times which thecandidate values have been selected. A temporal layer may mean decodinglevels or decoding orders of images included in a GOP.

According to an embodiment, the prediction decoder 2150 may obtaininformation of indexes respectively assigned to candidate values from abitstream corresponding to a level of an image, a slice, a tile, or ablock, and assign the indexes to the candidate values in unit of ablock, a tile, a slice, or an image according to the obtainedinformation. Herein, a block may include a largest coding unit, a codingunit, or a transform unit.

According to an embodiment, the prediction decoder 2150 may assign anindex (for example, an index of 0) of a predetermined value to apredetermined candidate value among the candidate values, and adaptivelyassign indexes to the remaining candidate values according to presetcriterion (for example, accumulative numbers of times which thecandidate values have been selected in previous blocks).

The prediction decoder 2150 may adaptively determine the number ofcandidate values included in a weight candidate group for each image,each slice, each tile, or each block. Herein, adaptively determining thenumber of candidate values means changing the number of the candidatevalues variously according to preset criterion, without determining thenumber of the candidate values to be the same number. Accordingly, whenthe number of candidate values included in a weight candidate group isdetermined in unit of an image, the prediction decoder 2150 may use,upon bi-directional prediction decoding of blocks included in a currentimage, a weight candidate group including 5 candidate values (forexample, 4, 5, 3, 10, −2), and use, upon bi-directional predictiondecoding of blocks included in a next image, a weight candidate groupincluding 3 candidate values (for example, 3, 4, 5). According to animplementation example, when the number of candidate values included ina weight candidate group is determined, the prediction decoder 2150 mayadaptively assign indexes to the corresponding candidate values, asdescribed above.

According to an embodiment, the prediction decoder 2150 may adaptivelydetermine the number of candidate values included in a weight candidategroup based on at least one of a POC of a first reference image and aPOC of a second reference image.

For example, the prediction decoder 2150 may adaptively determine thenumber of candidate values included in a weight candidate group, basedon at least one of a result of comparison between the POC of the firstreference image and the POC of the current image and a result ofcomparison between the POC of the second reference image and the POC ofthe current image.

For example, the prediction decoder 2150 may adaptively determine thenumber of candidate values included in a weight candidate group inconsideration of whether the POC of the first reference image and thePOC of the second reference image are greater or smaller than the POC ofthe current image. When both the POC of the first reference image andthe POC of the second reference image are smaller than or equal to thePOC of the current image, the prediction decoder 2150 may determine thenumber of candidate values included in a weight candidate group to be m(m is a natural number), and, when at least one of the POC of the firstreference image and the POC of the second reference image is greaterthan the POC of the current image, the prediction decoder 2150 maydetermine the number of candidate values included in a weight candidategroup to be n (n is a natural number which is different from m). Forexample, when m is 5, the candidate values included in the weightcandidate group may be 4, 5, 3, 10, and −2, and, when n is 3, thecandidate values included in the weight candidate group may be 3, 4, and5.

For example, the prediction decoder 2150 may adaptively determine thenumber of candidate values included in a weight candidate group based oncount information included in a bitstream.

Also, for example, the prediction decoder 2150 may adaptively determinethe number of candidate values included in a weight candidate groupaccording to a type of a current image. When a current image is a presettype, the prediction decoder 2150 may determine the number of candidatevalues included in a weight candidate group to be 5 (for example, 4, 5,3, 10, −2), and, when the current image is not the preset type, theprediction decoder 2150 may determine the number of candidate valuesincluded in a weight candidate group to be 3 (for example, 3, 4, and 5).

In regard of Equation 1, it has been described that, when a firstreference block is combined with a second reference block, a candidatevalue is determined according to a weight index, a pair value isdetermined from the candidate value, and the candidate value and thepair value are applied to the first reference block and the secondreference block, respectively. When the candidate value is w1, the pairvalue w0 may be 8-w1. Referring to the candidate values shown in FIG. 24, it is seen that the pair value w0 is also included as a candidatevalue in the weight candidate group. For example, when a candidate valueindicated by a weight index is 5, a pair value of 5 is 3 (8−5), and 3 isalso included as a candidate value in a table shown in FIG. 24 . Also,when a candidate value indicated by a weight index is 10, a pair valueof 10 is −2 (8−10), and −2 is also included as a candidate value in thetable.

That is, 5 is paired with 3 and 10 is paired with −2, among thecandidate values shown in FIG. 24 . When 5 or 10 is selected from amongthe candidate values, 3 and −2 may be always deduced as pair values. Byincluding 3 and −2 in the weight candidate group, various combinationsof candidate values to be applied to the first reference block and thesecond reference block may be determined. However, a large number ofcandidate values included in a weight candidate group may be adisadvantage in view of a bitstream. The reason may be because a largernumber of candidate values included in a weight candidate group resultsin a larger number of binary values respectively representing candidatevalues.

According to an embodiment, the prediction decoder 2150 may include onlycandidate values not being in a pair relationship in the weightcandidate group. FIG. 28 is a table representing candidate valuesincluded in a weight candidate group and weight indexes and binarystrings corresponding to the candidate values. Comparing with FIG. 24 ,FIG. 28 shows that 3 paired with 5 and −2 paired with 10 do not exist inthe weight candidate group. According to an implementation example, 3and −2, instead of 5 and 10, may be included in the weight candidategroup. Because a total number of candidate values is 3, a greatest indexmay be expressed with three binary values, and accordingly, the numberof bits required to express a weight index may be reduced.

When candidate values included in a weight candidate group are not in apair relationship, the prediction decoder 2150 may select which ones ofa candidate value indicated by a weight index and a pair value of thecandidate value are respectively applied to the first reference blockand the second reference block. More specifically, when preset criterionis satisfied, the prediction decoder 2150 may apply a candidate valueindicated by a weight index to the first reference block and a pairvalue of the corresponding candidate value to the second referenceblock. In contrast, when the preset criterion is not satisfied, theprediction decoder 2150 may apply the candidate value indicated by theweight index to the second reference block and a pair value of thecorresponding candidate value to the first reference block.

According to an embodiment, the prediction decoder 2150 may select whichones of a candidate value and a pair value are respectively applied tothe first reference block and the second reference block, based on a POCof the first reference image and a POC of the second reference image.For example, the prediction decoder 2150 may apply a candidate value toa reference block in a reference image having a POC having a greaterdifference from a POC of a current image among the first reference imageand the second reference image, and apply a pair value to a referenceblock in the other reference image. As another example, the predictiondecoder 2150 may apply a candidate value to a reference block in areference image having a POC having a smaller difference from a POC of acurrent image among the first reference image and the second referenceimage, and apply a pair value to a reference block in the otherreference image. As still another example, the prediction decoder 2150may apply a candidate value to a reference block in a reference imagehaving a greater POC among the first reference image and the secondreference image, and apply a pair value to a reference block in theother reference image. In contrast, the prediction decoder 2150 mayapply a candidate value to a reference block in a reference image havinga smaller POC among the first reference image and the second referenceimage, and apply a pair value to a reference block in the otherreference image. As another example, the prediction decoder 2150 mayapply a greater value of a candidate value and a pair value to areference block in a reference image having a POC having a smallerdifference from a POC of a current image among the first reference imageand the second reference image, and apply a smaller value of thecandidate value and the pair value to a reference block in a referenceimage having a POC having a greater difference from the POC of thecurrent image among the first reference image and the second referenceimage. As still another example, the prediction decoder 2150 may apply agreater value of a candidate value and a pair value to a reference blockin a reference image having a POC having a greater difference from a POCof a current image among the first reference image and the secondreference image, and apply the pair value to a reference block in theother reference image, and apply a smaller value of the candidate valueand the pair value to a reference block in a reference image having aPOC having a smaller difference from the POC of the current image amongthe first reference image and the second reference image.

FIG. 29 is a flowchart illustrating an image decoding method accordingto an embodiment.

In operation S2910, the image decoding apparatus 2100 may obtain a firstreference block in a first reference image and a second reference blockin a second reference image for bi-directional prediction of a currentblock. The image decoding apparatus 2100 may obtain, from a bitstream,information indicating a first reference image included in a list 0 anda second reference image included in a list 1, and select the firstreference image and the second reference image according to the obtainedinformation. Also, the image decoding apparatus 2100 may determine afirst motion vector indicating the first reference block in the firstreference image and a second motion vector indicating the secondreference block in the second reference image.

In operation S2920, the image decoding apparatus 2100 may obtain weightinformation included in the bitstream, and perform entropy decoding onthe weight information to obtain a weight index, in operation S2930.

The image decoding apparatus 2100 may reconstruct a first binary valuecorresponding to the weight index according to a context model, andreconstruct the remaining binary values according to a bypass method.

In operation S2940, the image decoding apparatus 2100 may combine thefirst reference block with the second reference block according to acandidate value indicated by the weight index among candidate valuesincluded in a weight candidate group, and reconstruct the current blockbased on the combined result of the first reference block and the secondreference block, in operation S2950.

The image decoding apparatus 2100 may determine the combined result ofthe first reference block and the second reference block as the currentblock, or the image decoding apparatus 2100 may determine the combinedresult of the first reference block and the second reference block as aprediction block and sum the prediction block with residual dataobtained from the bitstream to reconstruct the current block.

The image decoding apparatus 2100 may adaptively determine the numbersand/or kinds of candidate values to be included in the weight candidategroup for each image, each slice, each tile, or each block, in order toselect the candidate value indicated by the weight index. To adaptivelydetermine the numbers and/or kinds of candidate values to be included inthe weight candidate group, the image decoding apparatus 2100 may usethe same method as that performed by the image encoding apparatus 3000which will be described later.

FIG. 30 is a block diagram illustrating a configuration of the imageencoding apparatus 3000 according to an embodiment.

Referring to FIG. 30 , the image encoding apparatus 3000 may include aprediction encoder 3010, an entropy encoder 3030, and a generator 3050.The prediction encoder 3010 and the entropy encoder 3030 may correspondto the encoder 20 shown in FIG. 2 , and the generator 3050 maycorrespond to the bitstream generator 210 shown in FIG. 2 . Also, theprediction encoder 3010 and the entropy encoder 3030 may respectivelycorrespond to the prediction encoder 2015 and the entropy encoder 2025shown in FIG. 20 .

The prediction encoder 3010 and the entropy encoder 3030, according toan embodiment, may be implemented as at least one processor. The imageencoding apparatus 3000 may include at least one memory (not shown)storing input/output data of the prediction encoder 3010, the entropyencoder 3030, and the generator 3050. Also, the image encoding apparatus3000 may include a memory controller (not shown) for controlling datainput/output of the memory (not shown).

The prediction encoder 3010 may determine a prediction mode of a currentblock. The prediction mode of the current block may include an intramode, an inter mode, a merge mode, etc. The inter mode, the merge mode,etc. may be a mode for predicting and encoding a current block based ona reference image to reduce temporal redundancy between images.According to an embodiment, to encode a current block based on areference image, a reference image (that is, uni-directional prediction)or two reference images (that is, bi-directional prediction) may beused.

When a current block is bi-directionally predicted, the predictionencoder 3010 may select a reference image of the current block fromamong lists 0 and 1. The prediction encoder 3010 may select a firstreference image from the list 0, and a second reference image from thelist 1.

Also, the prediction encoder 3010 may search for a first reference blockfor predicting the current block in the first reference image, andsearch for a second reference block in the second reference image. Theprediction encoder 3010 may select the first reference block and thesecond reference block capable of generating a block having a smallestdifference from the current block from the first reference image and thesecond reference image. As described above, the prediction encoder 3010may generate a motion vector candidate list by using motion vectors of atemporal block temporally related to the current block and a spatialblock spatially related to the current block, determine a first motionvector and a second motion vector from among motion vector candidatesincluded in the motion vector candidate list, and obtain the firstreference block indicated by the first motion vector and the secondreference block indicated by the second motion vector.

After the first reference block and the second reference block areobtained, the prediction encoder 3010 may select a candidate value to beused to combine the first reference block with the second referenceblock from among candidate values included in a weight candidate group.

The prediction encoder 3010 may adaptively assign, before selecting thecandidate value, indexes to the candidate values included in the weightcandidate group.

According to an embodiment, the prediction encoder 3010 may adaptivelyassign the indexes to the candidate values according to accumulativenumbers of times which the candidate values have been selected, forbi-directional prediction of previous blocks encoded earlier than thecurrent block. For example, the prediction encoder 3010 may assignindexes of smaller magnitudes to candidate values selected the greateraccumulative number of times. As shown in FIG. 27 , when a candidatevalue 4 has been selected 19 times, a candidate value 5 has beenselected 6 times, a candidate value 3 has been selected 12 times, acandidate value 10 has been selected 3 times, and a candidate value −2has been selected 7 times in previous blocks, indexes of valuesincreasing in an order of the candidate values 4, 3, −2, 5, and 10 maybe respectively assigned to the corresponding candidate values.

According to an embodiment, the prediction encoder 3010 may newly assignindexes to candidate values for each image, each slice, each tile, oreach block. Herein, a block may include a largest coding unit, a codingunit, or a transform unit.

For example, when indexes are newly assigned to candidate values in unitof an image, the prediction encoder 3010 may calculate, before decodinga current image including a current block, accumulative numbers of timeswhich the candidate values have been selected for bi-directionalprediction of previous blocks included in previous images, and assignindexes to candidate values to be used for bi-directional prediction ofblocks in the current image according to the accumulative numbers oftimes. According to another example, when indexes are newly assigned tocandidate values in unit of a slice, the prediction encoder 3010 maycalculate, before decoding a current slice including a current block,accumulative numbers of times which the candidate values have beenselected for bi-directional prediction of previous blocks included inprevious slices, and assign indexes to candidate values to be used forbi-directional prediction of blocks in a current slice according to thecalculated accumulative numbers of times. According to another example,when indexes are newly assigned to candidate values in unit of a tile,the prediction encoder 3010 may calculate, before decoding a currenttile including a current block, accumulative numbers of times which thecandidate values have been selected for bi-directional prediction ofprevious blocks included in previous tiles, and assign indexes tocandidate values to be used for bi-directional prediction of blocks inthe current tile according to the calculated accumulative numbers oftimes.

According to an embodiment, the prediction encoder 3010 may selectbi-directionally predicted previous blocks based on the first referenceimage and the second reference image used for bi-directional predicationof the current block among the previous blocks, and assign indexes tocandidate values according to accumulative numbers of times which thecandidate values have been selected for bi-directional prediction of thecorresponding previous blocks. In this case, the entropy encoder 3030may perform entropy encoding on information (for example, ref_idx)indicating reference images used for bi-directional prediction of thecurrent block, and then perform entropy encoding on a weight index (forexample, bcw_idx).

According to an embodiment, the prediction decoder 2150 may select aprevious image having the same temporal layer as that of the currentimage in a GOP including the current image from among previous images,and assign indexes to candidate values according to accumulative numbersof times which the candidate values have been selected forbi-directional prediction of previous blocks included in the selectedprevious image.

According to an embodiment, the prediction encoder 3010 may assign anindex (for example, an index of 0) of a predetermined value to apredetermined candidate value among the candidate values, and adaptivelyassign indexes to the remaining candidate values according to presetcriterion (for example, accumulative numbers of times which thecandidate values have been selected in previous blocks).

The prediction encoder 3010 may adaptively determine the number ofcandidate values included in a weight candidate group for each image,each slice, each tile, or each block. When the number of candidatevalues included in a weight candidate group is determined in unit of animage, the prediction encoder 3010 may use, upon bi-directionalprediction encoding of blocks included in a current image, a weightcandidate group including 5 candidate values (for example, 4, 5, 3, 10,−2), and use, upon bi-directional prediction encoding of blocks includedin a next image, a weight candidate group including 3 candidate values(for example, 3, 4, 5).

According to an embodiment, the prediction encoder 3010 may adaptivelydetermine the number of candidate values included in a weight candidategroup based on at least one of a POC of the first reference image and aPOC of the second reference image.

For example, the prediction encoder 3010 may determine the number ofcandidate values included in a weight candidate group, based on at leastone of a result of comparison between the POC of the first referenceimage and the POC of the current image and a result of comparisonbetween the POC of the second reference image and the POC of the currentimage.

For example, the prediction encoder 3010 may determine the number ofcandidate values included in a weight candidate group in considerationof whether the POC of the first reference image and the POC of thesecond reference image are greater or smaller than the POC of thecurrent image. When both the POC of the first reference image and thePOC of the second reference image are smaller than or equal to the POCof the current image, the prediction encoder 3010 may determine thenumber of candidate values included in the weight candidate group to bem (m is a natural number), and, when at least one of the POC of thefirst reference image and the POC of the second reference image isgreater than the POC of the current image, the prediction encoder 3010may determine the number of candidate values included in the weightcandidate group to be n (n is a natural number that is different fromm).

Also, for example, the prediction encoder 3010 may adaptively determinethe number of candidate values included in the weight candidate groupaccording to a type of a current image. When a current image is a presettype, the prediction encoder 3010 may determine the number of candidatevalues included in a weight candidate group to be 5 (for example, 4, 5,3, 10, −2), and, when the current image is not the preset type, theprediction encoder 3010 may determine the number of candidate valuesincluded in a weight candidate group to be 3 (for example, 3, 4, 5).

After a candidate value for bi-directional prediction of the currentblock is selected, the prediction encoder 3010 may combine the firstreference block with the second reference block according to theselected candidate value to thereby encode the current block. Theprediction encoder 3010 may combine the first reference block with thesecond reference block according to Equation 1. The prediction encoder3010 may determine the combined result of the first reference block withthe second reference block to be the current block, and transferinformation (for example, information indicating the first referenceimage and the second reference image, information indicating a firstmotion vector and a second motion vector, etc.) for obtaining the firstreference block and the second reference block, and a weight index tothe entropy encoder 3030. Alternatively, the prediction encoder 3010 maydetermine the combined result of the first reference block with thesecond reference block to be a prediction block, and transfer residualdata between the prediction block and the current block, information forobtaining the first reference block and the second reference block, anda weight index to the entropy encoder 3030.

According to an embodiment, the prediction encoder 3010 may include onlycandidate values not being in a pair relationship in a weight candidategroup. By comparing candidate values shown in FIG. 28 with candidatevalues shown in FIG. 24 , it is seen that 3 (for example, a case of pairvalue=8−candidate value) being in a pair relationship with 5 and −2being in a pair relationship with 10 do not exist as candidate values inFIG. 28 . According to an implementation example, 4, 3, and −2 may beincluded as candidate values in the weight candidate group.

When the candidate values included in the weight candidate group are notin a pair relationship, the prediction encoder 3010 may select whichones of a candidate value selected from among the candidate values and apair value of the candidate value are respectively applied to the firstreference block and the second reference block. More particularly, whenpreset criterion is satisfied, the prediction encoder 3010 may apply thecandidate value to the first reference block and the pair value of thecorresponding candidate value to the second reference block. Incontrast, when the preset criterion is not satisfied, the predictionencoder 3010 may apply the candidate value to the second reference blockand the pair value of the corresponding candidate value to the firstreference block.

According to an embodiment, the prediction encoder 3010 may select whichones of the candidate value and the pair value are respectively appliedto the first reference block and the second reference block, based on aPOC of the first reference image and a POC of the second referenceimage. For example, the prediction encoder 3010 may apply the candidatevalue to a reference block in a reference image having a POC having agreater difference from a POC of the current image among the firstreference image and the second reference image, and apply the pair valueto a reference block in the other reference image. As another example,the prediction encoder 3010 may apply the candidate value to a referenceblock in a reference image having a POC having a smaller difference fromthe POC of the current image among the first reference image and thesecond reference image, and apply the pair value to a reference block inthe other reference image. As still another example, the predictionencoder 3010 may apply the candidate value to a reference block in areference image having a greater POC among the first reference image andthe second reference image, and apply the pair value to a referenceblock in the other reference image. In contrast, the prediction encoder3010 may apply the candidate value to a reference block in a referenceimage having a smaller POC among the first reference image and thesecond reference image, and apply the pair value to a reference block inthe other reference image. As another example, the prediction encoder3010 may apply a greater one of the candidate value and the pair valueto a reference block in a reference image having a POC having a smallerdifference from the POC of the current image among the first referenceimage and the second reference image, and apply the pair value to areference block in the other reference image, and apply a smaller one ofthe candidate value and the pair value to a reference block in areference image having a POC having a greater difference from the POC ofthe current image among the first reference image and the secondreference image. As still another example, the prediction encoder 3010may apply a greater one of the candidate value and the pair value to areference block in a reference image having a POC having a greaterdifference from the POC of the current image among the first referenceimage and the second reference image, and apply the pair value to areference block in the other reference image, and apply a smaller one ofthe candidate value and the pair value to a reference block in areference image having a POC having a smaller difference from the POC ofthe current image among the first reference image and the secondreference image.

The entropy encoder 3030 may perform entropy encoding on informationtransferred from the prediction encoder 3010. As described above, theinformation transferred from the prediction encoder 3010 may include atleast one of a prediction mode of a current block, informationindicating a reference image, information indicating a motion vector,residual data between a prediction block and the current block, and aweight index.

The entropy encoder 3030 may binarize the weight index for entropyencoding of the weight index, perform arithmetic encoding on a firstbinary value of a binary string corresponding to the weight indexaccording to a context model, and perform arithmetic encoding on theremaining binary values according to a bypass method.

The entropy encoder 3030 will be described in detail with reference toFIG. 31 .

FIG. 31 illustrates a configuration of the entropy encoder 3030according to an embodiment.

Referring to FIG. 31 , the entropy encoder 3030 according to anembodiment may include a binarizer 3031, a context modeler 3032, and abinary arithmetic coder 3035. The binary arithmetic coder 3025 mayinclude a regular encoder 3033 and a bypass encoder 3034.

Because syntax elements (information transferred from the predictionencoder 3010) input to the entropy encoder 3030 may be not binaryvalues, the binarizer 3031 may binarize the syntax elements and output abinary string configured with binary values of 0 or 1. The binary stringmay be arithmetically encoded through CABAC.

The binarizer 3031 may apply one of fixed length binarization, truncatedrice binarization, k-th order exp-Golomb binarization, and Golomb-ricebinarization according to types of the syntax elements to transformvalues of the syntax elements to binary values of 0 and 1. According toan embodiment, the binarizer 3031 may binarize the weight index by theTruncated Rice binarization.

The binary values output from the binarizer 3031 may be arithmeticallyencoded by the regular encoder 3033 or the bypass encoder 3034. Whichone of the regular encoder 3033 or the bypass encoder 3034 will encodethe binary values may be determined according to the types of the syntaxelements.

The regular encoder 3033 may perform arithmetic encoding on the binaryvalues based on a probability model determined by the context modeler3032. The context modeler 3032 may provide a probability model for acurrent binary value to the regular encoder 3033. More particularly, thecontext modeler 3032 may determine a probability of a preset binaryvalue based on a previously encoded binary value, update a probabilityof a binary value used to encode the previous binary value, and outputthe updated probability to the regular encoder 3033.

According to an embodiment, the context modeler 3032 may determine acontext model by using a context index ctxldx, and determine anoccurrence probability of an LPS or an MPS of the context model andinformation valMPS about which one of 0 and 1 corresponds to the MPS.According to another embodiment, the context modeler 3032 may determinea probability (P(1) representing an occurrence probability of, forexample, “1”) of a predetermined, preset binary value, based onpreviously decoded binary values, without distinguishing an MPS from anLPS, based on previously encoded binary values, and provide theprobability of the preset binary value to the regular encoder 3033.

The regular encoder 3033 may perform binary arithmetic encoding based onthe probability of the preset binary value provided from the contextmodeler 3032 and a binary value that is to be currently encoded. Morespecifically, the regular encoder 3033 may determine an occurrenceprobability P(1) of “1” and an occurrence probability P(0) of “0” basedon the probability of the preset binary value provided from the contextmodeler 3032. Also, the regular encoder 3033 may split a preset rangerepresenting a probability section according to the occurrenceprobabilities P(0) and P(1) of “0” and “1”, and output a binary value ofa representative value belonging to a split section corresponding to thebinary value that is to be currently encoded to thereby perform binaryarithmetic encoding.

The bypass encoder 3034 may fix the occurrence probability P(1) of “1”and the occurrence probability P(0) of “0” to a predetermined value, forexample, 0.5, split a preset range representing a probability sectionaccording to the occurrence probabilities P(0) and P(1), and output abinary value of a representative value belonging to a split sectioncorresponding to a binary value that is to be currently encoded. Thebypass encoder 3034 may perform high-speed arithmetic encoding becauseof using no context model, unlike the regular encoder 3033.

According to an embodiment, the entropy encoder 3030 may perform, uponentropy encoding of a weight index, arithmetic encoding on a firstbinary value of binary values corresponding to the weight index based ona context model, and perform arithmetic encoding on the remaining binaryvalues by a bypass method. In other words, a first binary value of abinary string corresponding to the weight index may be arithmeticallyencoded by the regular encoder 3033, and the remaining binary values maybe arithmetically encoded by the bypass encoder 3034. The reason may bebecause i) when a selection probability of a candidate value indicatedby a weight index 0 is highest, it is efficient to arithmetically encodea first binary value by accumulating changes of a probability of 0 and aprobability of 1 according to a context model, and ii) because selectionprobabilities of candidate values indicated by other weight indexesexcept for 0 are lower than the selection probability of the candidatevalue indicated by the weight index 0, and accordingly, it is moreefficient to arithmetically encode binary values by the bypass methodthan to reconstruct the binary values based on a context model.

Referring to FIG. 23 , it is seen that a binary value (that is, a firstbinary value) having binldx of 0 among binary values of Bcw_idxrepresenting weight indexes is encoded according to context of 0, andbinary values having binldx that is greater than 0 are encoded by thebypass method.

The generator 3050 may generate a bitstream including data output fromthe entropy encoder 3030. As described above, the bitstream may includeinformation to be used to reconstruct the current block.

FIG. 32 is a flowchart illustrating an image encoding method accordingto an embodiment.

In operation S3210, the image encoding apparatus 3000 may obtain a firstreference block in a first reference image and a second reference blockin a second reference image for bi-directional prediction of a currentblock. The image encoding apparatus 3000 may select the first referenceimage included in a list 0 and the second reference image included in alist 1, and determine a first motion vector indicating the firstreference block in the first reference image and a second motion vectorindicating the second reference block in the second reference image.

In operation S3220, the image encoding apparatus 3000 may select acandidate value that is to be used to combine the first reference blockwith the second reference block from among candidate values included ina weight candidate group. To select a candidate value to be applied tothe first reference block or the second reference block, the imageencoding apparatus 3000 may adaptively determine the numbers and/orkinds of candidate values to be included in a weight candidate group foreach image, each slice, each tile, or each block.

In operation S3230, the image encoding apparatus 3000 may performentropy encoding on a weight index indicating the candidate valueselected from among the candidate values included in the weightcandidate group. The image encoding apparatus 3000 may encode a firstbinary value corresponding to the weight index according to a contextmodel, and encode the remaining binary values according to a bypassmethod.

In operation S3240, the image encoding apparatus 3000 may generate abitstream including data generated as a result of the entropy encoding.The bitstream may further include at least one of informationrepresenting a prediction mode of the current block, informationindicating a reference image, information indicating a motion vector,and residual data, in addition to weight information obtained as theresult of arithmetic encoding on the weight index.

Meanwhile, the embodiments of the disclosure may be written as programsthat are executable on a computer, and the programs may be stored in amedium or a computer program product.

The medium or the computer program product may continuously store thecomputer-executable programs, or temporarily store thecomputer-executable programs for execution or downloading. Also, themedium or the computer program product may be any one of variousrecording media or storage media in which a single piece or plurality ofpieces of hardware are combined, and the medium or the computer programproduct is not limited to those directly connected to a certain computersystem, but may be distributed on a network. Examples of the medium orthe computer program product include magnetic media (e.g., a hard disk,a floppy disk, and a magnetic tape), optical recording media (e.g.,compact disc-read only memory (CD-ROM) and a digital versatile disc(DVD)), magneto-optical media (e.g., a floptical disk), read only memory(ROM), random access memory (RAM), a flash memory, etc., which areconfigured to store program instructions. Also, other examples of themedium or the computer program product include recording media andstorage media managed by application stores distributing applications orby websites, servers, and the like supplying or distributing othervarious types of software.

So far, the technical idea of the disclosure has been described based onthe preferred embodiments, however, the technical idea of the disclosureis not limited to the above-described embodiments, and variousmodifications and changes are possible within the scope of the technicalidea of the disclosure by persons of ordinary skill in the art.

1. An image decoding method using bi-prediction, the image decodingmethod comprising: obtaining, from a bitstream, weight information andresidual data for a current block; obtaining a weight index by entropydecoding the weight information; obtaining a first reference block in afirst reference image and a second reference block in a second referenceimage, for bi-prediction of the current block; combining the firstreference block with the second reference block according to a candidatevalue indicated by the weight index among candidate values included in aweight candidate group; and reconstructing the current block based on aresult of the combining and the residual data, wherein a first binaryvalue corresponding to the weight index is entropy-decoded based on acontext model, wherein a remaining binary value corresponding to theweight index is entropy-decoded by a bypass method, and wherein a numberof candidate values indicatable by the weight index in the weightcandidate group when a picture order count (POC) of the first referenceimage and a POC of the second reference image are smaller than or equalto a POC of a current image is larger than a number of candidate valuesindicatable by the weight index in the weight candidate group when thePOC of the first reference image and the POC of the second referenceimage are greater than the POC of the current image.
 2. An imageencoding method using bi-prediction, the image encoding methodcomprising: obtaining a first reference block in a first reference imageand a second reference block in a second reference image, forbi-prediction of a current block; selecting a candidate value forcombining the first reference block with the second reference block fromamong candidate values included in a weight candidate group; obtainingresidual data using a result of the combining the first reference blockwith the second reference block, and the current block; performingentropy encoding on a weight index indicating the selected candidatevalue; and generating a bitstream including weight information obtainedas a result of the entropy encoding, and the residual data, wherein afirst binary value corresponding to the weight index is entropy-encodedbased on a context model, wherein a remaining binary value correspondingto the weight index is entropy-encoded by a bypass method, and wherein anumber of candidate values indicatable by the weight index in the weightcandidate group when a picture order count (POC) of the first referenceimage and a POC of the second reference image are smaller than or equalto a POC of a current image is larger than a number of candidate valuesindicatable by the weight index in the weight candidate group when thePOC of the first reference image and the POC of the second referenceimage are greater than the POC of the current image.
 3. A non-transitorycomputer-readable medium for recording a bitstream, the bitstreamcomprising: weight information; and residual data, wherein the residualdata is obtained by: selecting a first reference block in a firstreference image and a second reference block in a second referenceimage, for bi-prediction of a current block; selecting a candidate valuefor combining the first reference block with the second reference blockfrom among candidate values included in a weight candidate group;obtaining the residual data using a result of the combining the firstreference block with the second reference block, and the current block,wherein a weight index indicating the selected candidate value isentropy encoded to obtain the weight information, wherein a first binaryvalue corresponding to the weight index is entropy-encoded based on acontext model, wherein a remaining binary value corresponding to theweight index is entropy-encoded by a bypass method, and wherein a numberof candidate values indicatable by the weight index in the weightcandidate group when a picture order count (POC) of the first referenceimage and a POC of the second reference image are smaller than or equalto a POC of a current image is larger than a number of candidate valuesindicatable by the weight index in the weight candidate group when thePOC of the first reference image and the POC of the second referenceimage are greater than the POC of the current image.